GeoffHa
The postmaster who does this service is rather unusual as the USPS does most of its philatelic sales through the Stamp Fulfillment Center - you can go to the USPS site, click Contact and look for the number for ordering stamps and stamp supplies. So due to the Oneco's postmaster's unusual status, he may have more restrictions than the Fulfillment Center. In addition the post office is located in a small suburb not far from the countryside in Connecticut. The nearest large town is Waterbury. He may have older customers used to using checks. The postal counter of his post office does takes cards but from in-person sales (as with practically all US post offices).
Just note the postmaster at Oneco sells most of what you get from the USPS Stamp Fulfillment center. The main benefit is you can get smaller quantities of coils for longer periods of times and, sometimes, commemorative stamps no longer sold online. He does not sell First Day Covers, stamp stationery and some of the collector folios that may have stamp proofs you find on the USPS site. On the other hand, the postmaster is a long-time philatelist who goes to shows and knows his stamps - something rare to find from a USPS employee. I highly recommend the service - especially as the postmaster is getting older so who knows if the service will be maintained years from now!
PS An unrelated sidebar only tangentially related to this discussion but there is a philatelic vendor you can buy online with your card or Paypal - it is one of the very few PRIVATE local posts left (and one that actual does postal services) in the US - Rattle snake Island in Ohio. If this interests you also, check their website for stamps and covers. I have had an email conversation with the one who runs this service and he does sell covers from 1967 - 1977 too but you have let him know on the site's contact form. Here is a link to their site -
https://www.rilp.org/P.S>S Their was a very active local post based in Hawaii from the late 1990's through 2014 and a few years ago you could buy stamps used or unused, booklets or singles, and fdcs but according to the site an auction house bought their entire inventory. THis study provides an overview of stamp issues fromthis former private local post.
https://www.pisc.org.uk/hawaii-post.htmlThe best you can do is call Oneco and see what options there may be.