[iRomain]

I don't know of any service but if you were to implement it, it's a cool small side/learning project. You can execute this logic either on-chain (smart contract) or off-chain (any programming language) or both. Here are some pointers:

Crypto (on-chain): This is pretty straightforward with a smart contract. The only thing is that you have to call your contract at the end of every month (that can be setup with a cron off-chain). The alternative is to offer a bounty for someone to make that call for you but you must find a way to advertise this bounty.

Non-crypto (off-chain): use PayPal/whatever api on a cron

Non-crypto (on-chain): You need to look for Paypal/whatever oracles (if there are any) or create your own

Crypto (off-chain): use a blockchain api and connect to a hosted node (eg infura) or run your own