You are right Tux I only know a little but I will share one of my assay bills to explain what I mean with minimum charge.
First I like to emphasize that acid test done by an assay office are very reliable, no matter what others my say or think .
Assay offices have to maintain there reputation as a trustworthy institute.
OK I always go in person to the assay office and do not sent items by mail because registered postal mail charges are not cheap and have to be taken into consideration.
The papers (borderel) I do together with the assay office assistance and discuss the possibilities.
I explained the cups, if not made out of silver, would be returned to the previous owner without any stamps.
The bill for two Indian cups sold as solid silver with no obvious hallmarks;

WAARBORGHOLLAND
‘Toetsonderzoek’ (assay charge) for 2 items 8.50
Kleine borderel kosten ( small permit paper) 9.00
Wacht behandelingskosten percentage (wait on result charge) 17,50
N.B. The 73.11 have nothing to do with the Indian cups.
Due to the fact the cups are without fake Dutch hallmarks or fake Convention marks the cups were labeled as non precious (onedel) only.
So we learned by practical experience assay costs at the end are an accumulated sum of all expenses made.
Oel