Puma return policy... 3 weeks for a credit

I bought some apparel from Puma during their big online sale and they were unfortunately the wrong size. I liked the clothes and wanted to exchange them for a size smaller, however Puma does not offer exchanges and only does returns. Since the sale is no longer going I cannot repurchase their merchandise at a discount. I sent the clothes back and noticed that it takes 2-3 weeks for Puma to credit my American Express card account for the return. This is really unacceptable and frustrating (I realize its my fault for buying the wrong size and buying without reading the return policy). I'm sure that they process the return immediately and then hang onto the money for as long as possible and put the credit through after 2 weeks and 6 days. I have already sent the merchandise back. Would it be appropriate to file a dispute with them through my credit card and stop payment to them? I'm sure I will be credited but I'm just very frustrated with the 3 week turnaround from them. Has anyone done something like this before?