A store that sells UPS packaging... They sent the visa application for me as well though UPS Express.
As KFDancer said, it's a bit odd that you were able to do it, since it's not supposed to be possible to pre-pay return international shipping, because you normally need to enter the size and weight of the package, and I think maybe also the address it is being mailed from and possibly even a mailing date... none of which you will know yet when you mail the package to Sheffield.
Normally what happens is you pay for the outbound shipping, then you include unpaid return packaging and either a UPS online waybill for the return shipping or just your UPS account number... and then your UPS account is billed for the return shipping when UKVI issue the visa and mail it back to you.