The tare
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What tare is
The weight of the empty container. Gross weight minus tare is net weight, and net weight is what the customer pays for.
Where it comes from in a refill shop
Either the customer's container is weighed empty at the counter before filling, or it carries a recorded tare from a previous visit, or it is a container you sold them whose tare you already know.
The third one is the cheapest
A shop that sells its own starter containers knows what each model weighs, which turns the tare from a weighing step into a lookup. That is an argument for selling a small number of models rather than many.
Which is a packaging decision with an operational payoff
Twenty different customer containers is twenty weighings. Three models plus a scale for anything else is a counter that moves.
Tare drifts
A jar with a damp label, a residue of last month's soap, a lid that was swapped. Recorded tares go stale and a shop that never re-weighs is slowly getting it wrong in one direction.
And the direction matters
If the recorded tare is lighter than the actual container, the customer is charged for the difference. That is the error that gets reported, and it is the one that accumulates.
Write the tare on the container
Which is the oldest answer and still the best one. A permanent mark on the base, in the unit the scale uses, readable when the container is wet.
The lid is part of it
Decide once whether the tare includes the lid, then do it the same way every time. A tare taken without the lid and a fill weighed with it on is a systematic overcharge.
Liquids can be sold by volume instead
Which avoids tare entirely: a measured pour of a known volume does not care what the container weighs. The method of sale section covers what is permitted.
And that is often the better answer for a small shop
No tare, no recorded weights, no drift. The trade-off is a dispensing method that measures volume accurately, which is its own equipment question.