Before that, answer here which providers do you usually use to get bitcoin - > USD prices?
As we will need these prices, we will need to select exchange rate provider for that.
For now we can already choose from a wide range of options, but we're going to use provider that everyone uses
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Why to choose a single exchange only when each of them have their own prices? Why not use services like preev or coindesk which gives you an average rate of BTC in USD?
People mostly use Bitmex, Binance and Bitfinex, so if you're still going to choose a specific exchange, go for one of them. As well, will your cart service also have a charge back or "Ask a refund" option if for any reason, a customer is not satisfied with the services/goods provided?