I remember @wiz talking about buying bitcoin on bisq with cash in the mail on a podcast years ago, but didn't understand how that worked and what the limits were. It's all pretty straightforward by the sound of it and the limits are $3,000 (if they haven't changed):
Next, enter the percent premium, or price, to pay but keep the total buy amount below $3,000. If the total is more than $3,000, then the Post Office cashier will be required to ask for identification. This is a trade-off that must be made for non-KYC Bitcoin. However, it is possible to acquire multiple US PMOs across a few days.
If this is your first time buying a US PMO, let me tell you; it is easy as pie. It is your right to use the USPS money service so exercise it. Simply, walk up to the cashier and tell them you want a money order for $XXX.XX. Complete the exchange and ask for a printed receipt (keep this for your records in case of arbitration). Should they ask you for identification, as nicely as possible tell them, “I do not have it on me and I am pretty sure I am not required to show identification for a US PMOs under $3,000.” Alternatively, try a different USPS or return later.