How to deal with wrongly-matched postcodes?

[Feldherren]

I'm more a customer on Shopify stores than a user, but for the past few months, every single Shopify-powered shop has matched my postcode wrong when I select my street address (and I have made certain that I'm selecting the correct town, street and number in the dropdown).

As an example, imagine I live in London on some random street and my postcode is SW2 9JW. I enter my house number and street name, and select the address from the dropdown. It autofills my address properly - correct city, county, et cetera - but instead of autofilling SW2 9JW, it autofills SW2 9TA.

I have lost deliveries this way and have had to contact the seller to get items re-sent or corrected at the delivery service, and whilst I usually remember to correct the postcode, I'm getting paranoid that I'll forget to do this when ordering something expensive. It's reaching the point where if I see the Shopify checkout, I'm less likely to buy items.

There doesn't seem to be a way to directly contact Shopify support; anything that looks like it'll work that way just takes you to a set of online help documents, and there don't seem to be any dealing with address-to-postcode matching. Does anyone have any ideas regarding how to get Shopify to look into wrongly-matched postcodes? Or where Shopify gets its address-postcode-matching data for the UK from, so I can try to chase it up closer to the source?