get a hold of a double or triple wall box ..... double's are a lot more common ... big enough for the amp and some packing ...I would find something that will give you a few inches around any part of the amp
pack it yourself ..... don't rely on the UPS store or anyone else ....
protect the front and back of the amp with extra panels of cardboard ...
wrap amp in a trashbag big enough to fit it ........ ( you have no idea if your box will sit in the rain at some point or if something is spilled )
and pack enough packing paper or bubble wrap IN THE BOX SO THE AMP CAN'T MOVE at all inside the box