Ignore these folks.
It looks like you used the Google Play Store, which is a legitimate way to purchase Credits, and they do cost more on there than via PayPal in the web version of the game because of the associated much higher fees Google charges.
I am going to send you an in-game message, but this is a semi-frequent issue (not sure what causes it to only sometimes happen) and we can manually issue your Credit.
In the future, for a faster response, please open a ticket in Discord via the #support channel