When your customer makes a purchase and wants to pay with Bitcoins, the conversion rate the customer gets is guaranteed by Coinify for 15 minutes. If the payment is completed after this time frame, the conversion rate will be calculated at the time the payment is finally paid and a late/extra payment will be created as a sub-payment to the original payment.
In other words, payments not paid within 15 minutes will expire. This means such payments will not update to 'paid' in the shop system, and it will show as expired on your Merchant Dashboard.
If the payment has been completed after 15 minutes, the expired payment will be attached to it as a sub-payment.
You can receive notifications for expired payments (and also partial payments) via callback or email, just like with regular payments. Go to 'Instant Payment Notifications' under the 'Integration Tools' menu to set it up.
! Please Note: It is up to you to arrange the operations with your customers in order to deliver the goods/services, due to the possibility of a price difference. You may have to contact them for a refund or credit--and may sometimes have to ask them to make the payment again.