Regards,
I experienced the paying and printing failure on the label flow earlier this week on 2 browsers - Firefox & Edge. It was sporadic but when it occurred I was unable to complete the label process until I tried the "Print labels in bulk" link on the label flow (green arrow - pic below).
The bulk label page has the same features as the single label flow page but the layout is different. You can also Remove items and print one label at a time or ALL your labels. You can Combine shipping on this page too. This work around may or may not work for everyone but its worth a try if you are in a pinch to get a label printed.
Regards,
Mr. L
