There are a few exceptions to using not so simple postage.
Your items up to £10 and 100 gms. shoukd be in one of the categories exempt.
Scroll to very end of this https://www.ebay.co.uk/help/selling/posting-items/setting-postage-options/simple-delivery?id=5575&st...
Collectables (e.g. Trading Cards, Card Games, Badges & Patches, Postcards)
You may need to play around a bit with sub categories to find one that works.
It is all down to using the right category and cards should be ok.