Why does an invoice expire after 15 minutes?

When a BitPay invoice is generated, it is only valid for 15 minutes to protect against the volatility of the cryptocurrency paying the invoice. During the 15 minute invoice window, the rate of the specific cryptocurrency is locked and guaranteed. After the invoice expires (after 15 minutes), it is necessary to generate a new BitPay invoice to try and pay again.

Please refer to How To Pay a BitPay Invoice.