I just joined last month for the first time, and tried to use my $5 coupon online, which gave me an error no matter what I tried to apply it to. I then tried using it in store, and got the same issue. They eventually just gave me $5 off, but not using the official coupon. The employees told me there was some glitch when you subscribe online, and to always do it in store. But who knows if that's just because they wanted commission.
They also told me I couldn't just buy $5 in Xbox/Switch/PSN credit, because that was a custom amount, and I had to do a minimum of $10, that they could then scan off a sheet stuck to the top of the desk, yet a friend told me he's had no problems with $5 eShop credit at the same store, so maybe I just went on a bad day.