Feels like I am getting cheated.

Got a mail from NGC saying that I owed them $14.50 in s&h from submission from July. This is the fourth time that NGC claimed I have underpaid my s&h, and frankly, I am getting annoyed with it. Whenever I charge the grading fee on my debit card, they AUTOMATICALLY charge me the extra cost of shipping (which I disputed everytime, and I actually received credits). But when I charge the grading fee to my credit card, they do not automatically get charged. Instead, I get an invoice from them. When I talked to a person from accounting department, he told me that they had to transfer to a different invoice number because of some reason, and blah, blah, blah. My gut tells me that they are cheating me. I wonder if it's time to move on to PCGS since PCGS graded coins seem to be valued higher than NGC graded coins.