Interestingly here in rural west Wales, Post Office facilties have improved a lot in the last year. My 'big little supermarket' is a Nisa store about 4 miles away. Last year the Post Office in the main street closed and they opened a cubicle in the suprmarket which was much easier for parking etc.
Earlier this year the Co-op took over the Nisa chain and have been slowly changing the product range and the hours of opening for the shop. As part of this they've increased the opening hours of the PO. Sorry I can't tell you the actual hours as they're not online and it's pretty much open when I go there. I think it's open Saturday afternoons. It's certainly open something like 10am-1pm on SUNDAY - and I actually used this last Sunday to send a parcel.