Researchers have found that the price you pay impacts how much you like it.
If you paid a lot of money for something, you would usually feel a lot better when using that item.
If you're given a book to read for free as a gift, you would probably not be as motivated to read it.
But, if you paid a lot of money for that limited edition book, you would enjoy that boo,k.
Next time if you end up charging a PREMIUM for your service, don't feel bad. Your client would probably VALUE your service even more.