AMAZON GLITCH: How do I remove UPS and FedEx from my shipping options?

An Amazon glitch is making me buy a label through ONLY FedEx, for $13. I am able to buy one, elsewhere, from USPS for only $4.78--BUT I NOT be PROTECTED, if the buyer is a scammer.

How can I remove UPS and FedEx as my shipping options? That way I can cancel the order and have the customer repurchase it (and I can ship it through Amazon USPS because it will be the only option).

Thank you everyone!