I had a similar issue. I was told mine was too thick to be sent as a letter and had to be rung as a package. I am going to have to figure out how to do this better.....maybe space the seeds so the envelope is not as thick? Any ideas?

make sure the bubble envelope is no thicker than 3/4 inches, has some flexibility to the package; I.E. it can bend some, and be sure that the seed packets stay in place and don't move around so that the thickness of the package does't change.

Easiest way to do this is to tape the packets down on a piece of paper that will fit inside the bubble envelope that you are sending the seeds in.

When I trade seeds I always ask the trader to send me a list of the seeds I am sending and it usually comes on a sheet of printer paper, the arrangement printed out on it.
I fold the paper in half or more if needed. Check of the seeds that I had put aside for the trade and then us some invisible type tape to tape them to the paper.

The other thing I use is a digital postal scale purchased at one of the office supply stores. it was quit cheap at the time I purchased it several years ago.

I weight the bubble envelope and know exactly how much the postage should be and if doing a seed offer, I request exactly the correct amount of stamps for the seeds.

I stopped a long long time ago, maybe more than 5 years ago with dealing with the postal clerks for the correct postage. It didn't matter, they always got it wrong, no matter how I argued and brought in their own info from the USPS website to prove I was correct.

Now I just weigh the bubble envelope, put on the postage and drop them in the mail box outside the post office. end of dealing with inside and no problems.

One thing that must be remembered though is that getting the postage correct on one end of the line doesn't always translate to the correct postage on the receiving end.
The post office on the receiving end of the bubble envelope can decide that the postage was incorrect and attempt to charge the member on the receiving end to pay the additional postage.
I always make sure that those receiving my bubble envelopes are aware that the correct postage was put on the envelope and if the post office on their end is saying it is incorrect then they have to show their post office the info from the website to show that it is correct and the PO is wrong.

This is an issue that we have had with the post office for at least 10 years now and it is not getting corrected no matter what has been tried.

Fran