1) You need to create an account on Boosty (Do this through a Google account, it works best with it)

2) If Boosty shown an error when purchasing, try to unlink and link again your card in your account settings

3) It is also worth checking if your bank is blocking the transfer, if everything is fine, try to send me a tips in several transfers so that you get the same amount that is in the post, also write what you wanted to buy and I will send you a link in a message

4) If after these steps you still have an error, try contacting Boosty support with your problem
[Update]

If, despite the above options, you are unable to buy a post, then keep in mind that payments from PayPal are currently not working, Boosty ONLY accepts cards