The relay is apparently rated for 10A @ 250VAC, so I think should be fine for the continuous current, but I guess the inrush current when the power supply is first switched on is what's causing the relay contacts to fuse.
I have another 4 of these relay boards spare, so I'm wondering if I can either a) Somehow limit the inrush current to the power supply so that I can just use one of these boards, or b) Figure out the inrush current so I can use a more suitable relay.
If I go with b) - does anyone know of a relay that would be up to the task? Ideally 5v with the same package as the ones on those boards so I can do a straight swap, but I'm guessing the ones that can handle the higher inrush current are probably a bigger package, right?
The reason I'm switching on the AC side instead of the 5V was really just for power saving, so that the power supply isn't on constantly, only when I want it on (when the Arduino signals D5 high).
The relay is sticking in the on position - the output stuck on closed and when the whole thing was unplugged (both USB and AC), I managed to give the relay a few taps and it eventually unstuck and opened up again.