My trusty kanji dictionary (compiled by Wolfgang Hadamitzky and Mark Spahn) interprets the Chinese character 預 (azu or yo) as "entrust or receive for safekeeping". Thus it makes sense that cashiers would employ it in their rituals.
Here are some other everyday examples of the character in use, which I have found around town:
預金 -- Yokin -- Bank account, deposit.
預かり所 -- Azukarisho -- Depository, warehouse.
手荷物一時預かり所 -- Tenimotsu ichiji azukarijo -- Place for temporary (lit. "one hour") handbaggage storage...
(Anyway, for more of my Japanese lessons and observations on the language, click here!)