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That is the padded flat rate envelope rate. 70lbs, 1 price.
You can order these envelopes for FREE.
https://store.usps.com/store/browse/...atMSS_FlatRate
They will deliver them to your house. I generally order 2 packages of 20 each about every 6 months. You would be surprised just how much you can cram in there and not come close to 70 lbs.
I am wondering if you purchased a ready-post brand envelope at the store? Those are NOT flat rate, don't look like priority, but are generally stocked right next to the priority/flat rate supplies. You would absolutely pay based on the weight in addition to buying the envelope itself.