BAU with SSVs at USPS in NorCal today. I cringed at the $2.20 fee, but it would still cost more in gas to drive to a friendly WM, so...

I did have an SSV rejected at WM a few weeks ago. When I checked the balance (I forget if it was online or by phone), there was a message that some merchants put a hold on the cards and that it would be available at such and such time which worked out to an hour after purchase. It appeared to be store-specific since other cards I bought at other stores in the same chain and from another chain didn't have holds.