Please be aware that every offer has limits beyond what is displayed in the thread. You should visit the site advertising and read any Terms of Service or Acceptable Use Policy they have in place. If you can't find answers on how they measure what you're purchasing, ask.
If the offer or request doesn't regard some type of hosting, you still need to research beyond what’s printed in the ad.
Research any provider to know what limits are in place, especially with any offer concerning disk space and/or bandwidth. These are generally defined in the TOS or AUP.
Read provider reviews. If you find no reviews, create a thread and ask for others' first-hand experience with the provider.
If you're unsure on the exact details of an offer - ask. Simply respond to this thread and ask the provider for clarification.
I went to your link and I don't understand exactly what you are offering? What do you consider as 1 modification? Your prices seem cheap, but I am not sure what you are offering. If I were you... I would list all the different things that you can modify, so potential customers get a better understanding of what you do.
clear soem stuff up, by mod I mean forum modifications. A extremly common term to anyoen who uses phpBB. 1 modification os one forum mod examples would be attachment mod, shop mod, points system mod, etc. Also, I have modded forums before including multipel of my own forums, and a soiple of other peoples.