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If your office is like the one where I work, they're probably short on box clerks on saturdays. Ours only work 6 hours a day, and only one comes in on weekends. The window clerk that comes in early on Saturday helps out, but sometimes there's just too much to get done.
It's almost certainly not a decision on the part of your office in order to cut down lines. It's probably just that they're short of staff to do the work. In 21 years in the PO, I've never worked in any office where they had enough people necessary to do the work. Management just doesn't want to shell out the money for extra people.