Testing a web shop and think you're done? Here are some test ideas you may or may not have covered yet (that will still not be exhaustive):
Tested across supported countries? What differences should there be? Currency? Date & time formats? Payment methods? Shipping methods? Regulations?
Tested across supported languages? Special character rendering, capitalisation & punctuation, impact on cross-browser font rendering.
Tested with touch navigation? Tested across supported resolutions?
Pay now versus pay later. Credit card versus bank transfer.
Check out as logged in user versus guest user. With and without address set on account. With old and new account (keep old accounts around).
Simple products. Configurable products. Bundles.
Printing a page. Letting a page time out. Navigating in landscape mode.
Out of stock product. Product with limited stock. Reserved product.
Test impact of rejected or accepted state on cookies loaded. Choice remembered when it should be?
Other products in cart from old session. Other products in cart without stock. Other products in cart from different warehouse(s).
Changes on main site also made on other sites when consistency is expected?
Impact of changes on whether user data is still tracked as it should be.
What safe shopping or other regulations need to still be safeguarded?
Impact of changes on page load time, Lighthouse score, transaction times of affected functionality.
Do changes have implications for data privacy or create security vulnerabilities?
Do changes violate your supported level of the Web Content Accessibility Guidelines?
Verify that the structured data has the expected properties and values.