Interesting conundrum - I'll have to have the mobile purchase flow re-checked. It should not sign you up for a recurring subscription. Personally, I have only tested that on the native desktop client.
If a phone is the only thing available to you as a device, you could still open a browser, go to albiononline.com, log in and purchase via that flow, instead of using the mobile client. In the meantime, I'll go prod people about mobile in-client purchases.
Edit: We've run internal tests and didn't encounter that particular problem. Would you DM me your character name and the server you play on, plus ideally some screenshots of the message you're receiving when trying to purchase these things?
Edit 2: Puzzle potentially solved - it's not really pertaining to the subscription part of it. Did you buy your 30 day subscription on a different platform? Maybe even the native desktop client? If you do, and THEN try to buy on a different platform, system thinks (even though it's not) "might be a subscription. Don't want double subscriptions on one character, pls fix.". Something for the tech people to look at. But in the meantime: purchase via website or normal desktop client, and you should be good.