You get an envelope of the appropriate size. You can get larger envelopes from the post office so you can buy just one, rather than a whole pack.
You put your tape in, addressed as normal, and you take it back to the post office, have them weigh it and they will give it the correct postage for it's weight, then you just pay them what they need.
I highly doubt you will need more than a couple bucks for postage, but to be on the save side, take $5.00 with you for postage fee.
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