I had this happen same day, don't know if it was a glitch, then my listing page kept freezing. So I got off the page and reentered the listing format again. What i discovered is that certain heavier boxes I created are defaulting now to Priority instead of USPS ground advantage which it had recommended before and wasnt offering me that as an option in the list. I am not sure of the exact perimeters but it some cases it thinks that it exceeds the limits for ground advantage which it did not or if the dollar value exceeded $200 but under $400.
Some I left as Priority, others I finally was able to remove the Priority and start over by retraining the system to accept ground advantage or UPS Ground as my first choice. Double check your item specifics for any size & weight typos & that the shipping tool didn't prefill a length or weight far greater than your actual object as I have seen that happen also with AI. I think the system may also be on overload with holiday shopping - hope this helps- if not perhaps contact the shipping dept at Ebay.
