Thanks for your responses

[GasHouseGang]

No, I've never seen that. I did a quick search and found the following:

There are a number of exceptions and exclusions to the general gifts rule. Postal Service employees may accept the following items:
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Snacks and beverages that are not offered as part of a meal.
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Items with little intrinsic value (i.e., greeting cards, plaques, pens, coffee mugs, etc.).
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Perishable items (i.e., flowers, chocolates, cookies, etc.); if the items are clearly worth more than $20, employees should share them with others in the Postal Service workplace.
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Items with a market (retail) value of $20 or less.
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Gifts motivated solely because of a personal relationship.
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Gifts for which the employee has paid market (retail) value.
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Gifts paid for by the Postal Service.

Postal Service employees may not accept cash - in any amount or form (bills, checks, money orders) - from an outside source.

So it sounds like they are breaking the rules by setting up a tip jar.