Just service with simple API. I want to allow clients to pay usd or bitcoin. Then after payment complete i need to add my custom value on my site client purse. Is there simple system for it? Back end server on java
Checkout the Coinbase API: https://developers.coinbase.com/
Coinbase is very well known and trusted.
Then for the USD option you could have another button that links to Paypal, Stripe, 2Checkout, etc..