This might be nitpicking, but I've found a typo in the callback status. The actual `invalid` callback comes out capitalized as the only one. [This is the status I'm mentioning](https://github.com/MobilePayDev/MobilePay-Invoice/blob/f1d20f053c961d6db5fbb70caba6127331f652d2/docs/api_reference.md?plain=1#L461). 
This might be nitpicking, but I've found a typo in the callback status.
The actual
invalidcallback comes out capitalized as the only one. This is the status I'm mentioning.