So my credit card company cashed my payment check on 12/8, still hadn't posted to my credit card on 12/15 So I called customer service. Sent them a copy of the cancelled/cashed check proving they received my money by upload and fax. Initially they told me they would have it fixed within 24-48 hours, I called today and now they are saying it should be resolved by 1/2/18. Is there any recourse I have, any law that they can take that long to apply the money to my account when they have proof they received it?
Assuming you are in the US (please tag your question with a location if this is incorrect), the Consumer Financial Protection Bureau exists to make sure you're treated fairly with this sort of matter.
You should let your credit card company know if your payment does not show on your statement. You can call the card company to try and resolve the issue quickly. However, in order to protect your rights you must send a written billing error notice
The date they have advised they will have it resolved by may be before or after your next statement is created.
If they have resolved the issue and correctly backdate the credit for your payment before the statement posts, then you shouldn't have to do anything - all the interest charges and fees should be correctly recalculated. That said, you should still check carefully to make sure everything was properly corrected and let them know if they missed anything.
If they do not resolve the issue until after your next statement has been created then you should expect to see a credit on the following statement, to offset any interest that you were incorrectly billed. They should also back off any fees that are a direct result of incorrectly dated payment receipts.
Most credit card companies have thousands of customers, so this probably isn't the first time this has happened and I expect they are following a well defined procedure to correct the problem. For example, they may have mis-transcribed your account number and the funds may just be sitting in a lock box waiting to be allocated to an account. Financial institutions in the US are periodically audited by the CFPB to make sure they aren't being lazy when it comes to this sort of problem.
Give your provider some time to resolve the issue, but don't be afraid to enlist the help of the CFPB if they are taking too long. In either case, if the payment is not shown on your next statement, with the correct date, it would be wise to send in the written billing error notice to protect your rights.