Authorize.net integration with custom PHP/MySQL website

I run a website that integrates 2 different types of billing with authorize.net: automated recurring billing subscriptions and one-time transactions. Often times I have customers who are billed both ways within a month - a set monthly fee, and then they purchase something supplemental to their subscription.

Everything is pretty much implemented and functional, but it isn't very polished. There are scenarios where cards are declined but the customer insists everything is right. I need someone to come in and fully assess the billing situation. I need someone to lay out exactly what information is passed between authorize.net and my server, what information about the customer that is stored in the database is used in the transaction for the recurring portion and the one-time portion, etc.

For example, currently I have a customer whose recurring billing is working but when they try to purchase something with a one-time transaction, it is being declined. I think it has something to do with a mismatch of information stored on the server vs. what authorize.net has stored for the subscription. This needs to be troubleshooted and fixed.

I want someone who has experience with authorize.net integration into PHP/MySQL websites, and also who has experience with authorize.net's ARB (automated recurring billing) module. This is a must.

I will also likely rely on this person to make a few modifications to the custom billing page where someone can upgrade from one package to another, etc.