@Nick ******
Well, I don't like paying these little extra fees either, but have noticed Lao tends to apply "overtime" fees when entering at certain times of day/days of the week (for which there should be a receipt, but isn't always). Also, it appears to me that some fees may be levied if arriving by car (I always arrive by car) but may or may not be, if you come by bus.
I don't want any issues with my car hence why I generally pay up without question (though these days, there's no question of being allowed entry if you come by car).
Pre Covid, there was a different system in place with officials having discretion to randomly deny your vehicle entry or restrict the time you're given on your entry permit to less than 30 days and/or restrict you to the province entered. These days, it's automatically done by a machine at customs. You no longer have to tell customs where you're going - if using a passport its an automatic 30 days for the car and you can exit at any border. For border pass holders, u have to exit at the same border.
Consequently, I always pay these small extra fees, which are usually no more than 100 Baht anyway.
On my last entry into Laos in a group of 4 (my family) it was just after 9pm, the border was less than an hour away from closing and the immigration guy wanted 100 Baht. Initially I understood him to mean 100 Baht per person, so I got out 400 Baht but he gave me back 300.
I recall several years ago, at another border (Phu Doo) I was charged 700 Baht for 4 passengers entering Laos.
The one time I definitely don't pay any extra is if some tout shadows me and tells me I need to pay such and such amount to the immigration officer. Then I know it's a scam.
It also depends on the country. Entering or departing Vietnam, I know there's nothing to pay, so I refuse. Since I don't enter Vietnam by private vehicle anyway (hardly anyone does as Vietnam makes it difficult to bring in a temporary import) then I don't need to be too cautious. In any case, I only pay immigration, not some tout.
I also definitely refuse scam health checks such as the one the Cambodians operated at some land borders before Covid, when only westerners and Africans were targeted.