When United States post offices opened for business Wednesday morning, May 20, all point-of-sale equipment had been reprogrammed to automatically calculate prices on customer purchases at the new postage rates.

The problem is, those new rates don’t go into effect until May 31. Many sales rung up at the post office window on Wednesday, however, were coming up with an incorrect total by applying those new rates prematurely.

Postal customers who contacted Linn’s reported that postal clerks were aware of the problem and were manually calculating fees to circumvent the inaccurate computations.

May 31 rates turn up in post offices on May 20