which btc payment gateway api easy to use?
Bitpay is probably the best and the most used one out there , there are both free and business plans (free doesn't offer that much) but business one is awesome for only 1% transaction fee , you could check the difference here :
https://bitpay.com/pricing , It's possible to use with WordPress , WHMCS , SHopify etc... and no big programming knowledge is required.