How to charge a fee to process Bitcoin payments in a mobile point of sale app? I have a mobile point of sale app for local businesses.  We want to integrate Bitcoin as a new payment processor for our merchants.  We currently charge $.30 per transaction for credit card swipes and we want to charge the Bitcoin equivalent. How do we grab a transaction fee for submitting the transactions on the merchants behalf via our app?  I mean, how do we do this in Bitcoin? Is there a property in BitPay we can use that says pay this API_Key this fee amount?
Firstly Bitpay and Bitcoin are not the same thing. 

BitPay is the Paypal equivalent of the Bitcoin world. You may be able to have a percentage of the transaction for yourself, but you will have to work out the business logic. Can't help you there.

Regarding fees on the Bitcoin network, please note that it is designed to be anti-fee, so if you do add fees you must bring something else to the table, but it sounds like you are setting yourself up as an intermediary between the merchant and Bitpay so I don't know exactly what that might be.

Bitcoin is anti-fee because even it's own transaction fees are subject to minimum payment amounts otherwise the fee is zero.

The Bitcoin Transaction fees themselves are not calculated on the amounts but are a function of the number and size of the inputs and outputs, the size of the transaction in kb and whether or not the person has decided to pay a fixed fee or not. 

That being said your merchants will not be happy if you apply a fixed fee to their transactions especially if the transaction amount is less than the fixed fee you want.