Julie here from Shopify Support. Great question.
When an app developer issues app credits, they'll be automatically applied towards any future Shopify App Store purchases. You'll see the credits appear on your Shopify subscription bill, which is emailed to you monthly. You can also access your bill in Settings > Billing.
Are you looking for an app to replace SMSBump? Let me know if you need any app suggestions and I'd be happy to offer a few!