SPS Commerce waste time and money

We were required to use the system to do business with one of our Customers. It is a tedious method. However, the vendor was paying for it so we played along. Once we were forced to move to pay to receive our orders and then other customers decided to use the system to send orders, it quickly became a problem. We pull orders, ship, and invoice and receive no error messages. Then a month and now up to six months later we are getting notices for change orders and or closed p.o.'s are showing up in the queue. Orders that have been processed and already been paid. We are spending too much time investigating and pulling old invoices, sending documentation and imputing info, and then having to send support emails of screenshots of the issues. We knew in 2010 that the program was glitchy and chose not to move to it with. Maybe it works for a large company that has products that cost pennies on the dollar and they can hire someone to sit and play with the system and send support emails all day, but for a small company that is receiving one p.o. a month or week from customer to have errors every time we get into the system is ridiculous. It seems like the industry partners are trying to tell me how to conduct my business, charge me for it and not give decent service for their product.