Sorry if this is a bit OT, but we discussed postage a few days ago.
I've just received a letter with a warning sticker wingeing that it was underpaid, though graciously saying they wouldn't surcharge me this time, but to be more careful in future. As the weight was well under
100gm, it must have been deemed too thick (>= 5mm). I measured it at 4mm when squashed, but when the envelope was allowed to "balloon" it teetered over the border. I guess the sorting machinery uses an optical sensor rather than a mechanical one. Has anyone been into a PO lately? What kind of gauge do they use there? If it's just a slot, then it seems that a letter can be accepted at one rate at the counter but surcharged at the sorting office. A nice little earner :-(Just to bring it on topic -- it looks like we need a diy design for a non-contacting thickness gauge.
Chris