oOnly pay the MSPA. It is a yearly membership due. Like my Medical cetification, I have to pay every year. They give you training and updates. It is true, you don't have to do it, but almost every company I am signed up with asks for it. As well as a few other certifications that don't charge you. See my blog on Delphi Forums from IrishRose42. I have a whole list there.
These companies are very professional. They don't want to turn in a poor quality report to their customer. Spelling and grammar are very important. Bonnie's right on that one. Always spell and grammar check your narratives. Cut and paste them into your word processor, run your check, cut and paste them back into your document. Keep your shop.
A trick I use is to pick a zipcode, and go to several site and link my shops for the day to the zipcode. Little travel means more profit! Go check out my list on my blog on Delphi. They are good companies!