There are a lot of options that you can make, depending on your ability to technology:
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https://btcpayserver.org/: The best way, gives you a lot of options, you better try them.
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https://coingate.com/bitcoin-pos: It is an excellent alternative and you can withdraw money to your account in addition to many options.
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https://bitpay.com/: One of the worst options.
Did I understand your point exactly? Many services offer you this feature.
Thank you for your answer, but it's not exactly answer my question. I know how to accept bitcoin payment and to use bitcoin gateway. I try to get information of the exact price of bitcoin compare to dollar when my customer made an order. Also, I want the customer to choose how much USD / BTC he want to pay. So if he choose he want to pay me 900$ he will get information of how much Bitcoin he need to send equal to 900$.