In short
Set the rules in Merchant Center and use the feed for exceptions. Account settings describe how your store ships generally, apply to the whole catalogue and can be changed in one place, which is what most stores need. The shipping attribute in the feed overrides those settings for individual items and is the right tool for oversized goods, freight, hazardous items and per-product surcharges. The two are designed to work together, with shipping labels connecting groups of products to the rate rules that apply to them.
Account settings are the default because they describe the store
Shipping in Merchant Center is expressed as services: a delivery country, a rate structure and a delivery time. That maps well onto how most stores actually operate, where a single set of rules covers nearly everything and only a handful of products behave differently.
The practical advantage is maintenance. When a carrier changes rates or you introduce a free shipping threshold, one change in the account updates every product, and there is no feed to regenerate and no risk of part of the catalogue carrying stale values.
Rate structures can be simple or detailed. A flat rate, a rate that varies by order value, a table keyed to weight, dimensions or destination, or carrier-calculated rates where the account supports them for your carriers. Free shipping above a threshold is expressed as a rate rule rather than as a separate service.
The requirement underneath all of it is accuracy. What you configure has to match what a shopper is charged at checkout for the same basket and destination. Advertised shipping that differs from actual shipping is a mismatch and a policy issue, and it is also the fastest way to lose a shopper at the last step.
When the feed is the right place
Use the shipping attribute in the feed when a product genuinely does not follow the store's normal rules. Furniture and appliances that ship by freight, items with an oversize surcharge, hazardous goods with restricted carriers, and anything shipped directly from a supplier at a different cost all qualify.
The attribute lets you state a cost for a specific country or region on that item, and the value you supply takes precedence over the account rules for it. That precedence is the reason to use it sparingly: every product carrying its own shipping cost is a product whose shipping stops updating when you change your rates.
There is a middle path that most large catalogues should prefer. Shipping labels let you tag groups of products in the feed, then build account-level rate rules that apply to those labels. Oversized items get one label, freight items another, standard goods none, and each label has its own rule in the account. You keep the maintainability of account settings while expressing genuine differences.
Dimensions and weight belong in the feed regardless, since weight-based and dimensional rate tables cannot work without them, and missing values there are a common cause of shipping being calculated wrongly.
Delivery times deserve as much attention as cost
Delivery speed is shown to shoppers and increasingly forms part of the question they ask, so the transit and handling settings are not an afterthought. Handling time is how long you take before dispatch, transit time is how long the carrier takes, and the two together produce the estimate that appears with your listing.
Set them honestly and per region. A store shipping across Canada with one national estimate will overstate speed to remote areas and understate it to nearby cities, and both cost something: the first in complaints and returns, the second in lost clicks.
Cut-off times and non-working days matter here. If orders placed after mid-afternoon ship the next business day, say so, and set the weekend and holiday behaviour to match how your warehouse actually operates. Stores that skip this consistently promise faster delivery in December than they can meet.
Returns settings sit alongside shipping and follow the same rule: configure them in the account, keep them consistent with the returns policy on the site, and update both together. Shoppers and shopping surfaces increasingly treat returns terms as part of the offer, and a policy stated in three places with three different windows is a problem waiting to surface.
Keeping it consistent, and what breaks
The failure mode is always the same: what the advertisement says differs from what the checkout charges. That happens when the account settings were configured once and the store's real rates moved on, when a promotion changed a threshold on the site but not in the account, or when a product carries a feed override that nobody has revisited.
Build a check into the routine. Once a month, take a handful of products including at least one oversize item, add each to a cart, choose a destination in each region you target, and compare the checkout cost and estimated delivery against what the account and feed say. It takes minutes and it catches nearly everything.
Watch the diagnostics for shipping-related issues and treat them as urgent, because they affect eligibility rather than just accuracy. A product that cannot be assigned a shipping cost for a targeted region is not eligible to serve there.
And when you expand targeting into a new country or province, update shipping before turning campaigns on. Adding a market without configuring delivery for it is the single most common reason a newly targeted region produces no impressions at all.
Related questions
What if the checkout calculates rates dynamically?
Represent the outcome as closely as you can with rate tables or carrier-calculated rates, and check the result against real baskets across your destinations. Where a genuinely variable rate cannot be represented, use the closest accurate approximation rather than the cheapest one, since understating cost leads to mismatches and abandoned checkouts.
Does a free shipping threshold need to be reflected in the feed?
Not usually. It is expressed as a rate rule in the account, based on order value, and applies across the catalogue. Add feed-level values only for products excluded from the threshold, such as oversized items, and keep those exclusions consistent with what the site tells shoppers at checkout.
Which wins when the feed and the account settings disagree?
The value supplied in the feed for that item takes precedence over the account rules. That is useful for exceptions and dangerous when forgotten, because those products stop responding to rate changes made in the account. Keep a record of which items carry overrides and review it whenever your shipping rates change.