Mailing costs for Bubble Envelopes (BE)
Bubble envelopes which are empty, or under 3/4" thick are classified as and to be charged as Large Envelopes and referred to as Flats) The contents (seed packs) should be taped in place, so the BE will be uniformly thick, and that the seeds don't shift, and then cause problems with the USPS equipment.
Large Envelopes rates (BE under 3/4" thick)
1 oz=$ .83
Other rates up to 13 OZ can be found at the link below. Anything over 13 OZ will then go as Priority mail (1 LB) and will be $4.80 unless one tells the clerks it is to go Parcel Post.
If there are numerous generous amounts of seeds sent, or if the seeds are bigger, the BE may be more than 3/4" thick, and then will go at Parcel Rate. It is not necessary to tape packets down if it is a bigger bulky envelope over 3/4" thick.
Other heavier (First Class) Parcel Rates can be found at the link below.
Here is a link that might be useful: First Class rates for Large Envelopes and Parcels