Dealing with the Post Office can be tough!
How many yards of fabric were in the envelope? We've been able to send 8 yards in an envelope. Since we don't quite trust the adhesive on the seal, we run a single row of clear tape along the join, just to be sure.
And we did have to have "a serious discussion" with the Post Office about how much we can put in the envelope. We've sent many thousands of envelopes out, and only once did we hear from a customer that they had the same problem you did.
What can be a problem, I think, is if too much tape was used to seal the envelope. The PO sometimes objects to that.