I have Netstamps and just hit the butttons and let it figure the postage out.
What are Netstamps?

If you were to send an empty bubble envelope to someone, what cost would 'it' figure you should affix?

All empty bubble envelopes should be going for 83 cents, with NO exceptions. If you are receiving in BE with postage due, you really should try and address the problem with your Post Master/Mistress. They may not know their employees are overcharging by demanding that postage is due.
For that matter, I have found that there are numerous Post Masters/Mistress/Supervisors out there that are unaware themselves that the majority of seed trades are to go as Large Envelope rates, as opposed to parcels.

Most seed trades are under 3/4" thick, flexible, and should be uniformly thick if the seeds have been taped down to a piece of paper to keep them from shifting and bunching up.

First Class Mail costs for Letters, Large Envelopes, and Packages/Parcels

Please note...LETTER rate does not apply because almost all BE are over 1/4 " thick, even if they are empty. The PO has a template to drop them through to see if they are under 1/4", or if the they must go up to the next tier of Large envelope.

Lastly...MOST Post Offices it seems are just charging Parcel Rate in error it seems.

If you want to assist in trying to get the problem solved across the country

Sue