<p>BTCPay relies on Bitcoin Average for getting crypto-currency to fiat rates</p>
<p>If you want BTCPay rate cache to be smaller than 15min, you should register to BitcoinAverage and get a paid API Key.</p>
<p>If your BTCPay server instance supports lot's of merchant or is used a lot, BitcoinAverage will rate limit your server, and invoice will be created using less accurate rates. (From Bitpay)<br /></p>
<p class="form-text text-muted">You can find the information on <a target="_blank" href="https://bitcoinaverage.com/en/apikeys">bitcoinaverage api key page</a></p>