there’s no easy solution to it yet, you can check with publisher/developer directly if there are other payment options. There’s a list of Developers that do this at [LIST] Paid apps that can be purchased from developer and used on /e/OS
The FAQ states
https://doc.e.foundation/support-topics/app_lounge.html
In case a user had purchased paid apps in a previous life with a Google Android phone, we also provide him/her the option to sign in with a Google Account. Please note: In order to purchase a paid app, users will need to use a Google account linked to another device that has Google Play installed.
An… innovative - but not effortless - way to get at that Google Play store is running an Android VM for purchases: How to purchase a paid app