An invoice for physical metal is not a formality. It is the record that connects a specific quantity of metal to a specific buyer, and its contents matter later.
Weight and fineness must be stated separately
Metal is sold by weight at a stated purity, and those are two different numbers. A hundred grams at one fineness is not the same holding as a hundred grams at another.
Good invoices state the gross weight, the fineness and, where the two differ, the fine metal content that results from multiplying them.
Without those figures written down, a later valuation depends on re-testing the item rather than on reading a document, which costs time and sometimes money.
Product identity should be unambiguous
A line reading simply "gold bar" leaves open which refiner produced it, which format it is and whether it carries recognised accreditation.
Naming the refiner, the format and the serial number where one exists lets a future buyer match the physical item to the paperwork without further verification.
Serial numbers matter most on larger cast and minted bars, where the number appears on the bar itself and on the assay card sealed with it.
Price should be broken into its parts
A single total conceals how much of the payment was metal and how much was premium. Both figures are useful afterwards.
An invoice that records the metal price used, the premium charged and any separate fees allows the buyer to compare the transaction against later quotes on a consistent basis.
It also makes the round trip calculable, since a buyback bid can be set against the premium originally paid rather than against a vague total.
Delivery and custody terms belong on the document
Where metal is shipped, the invoice should say when title passes and who carries the risk while the parcel is in transit.
Where metal is stored by the seller, it should state whether the holding is allocated to the buyer as identified items or held as a general claim against a pool.
That distinction changes what the buyer owns in the event of the storage provider failing, so it is worth having in writing rather than in a website description.
Records support insurance and estates
Insurers assessing a holding want evidence of what exists and what it cost, and an itemised invoice supplies both without argument.
The same documents make a holding legible to anyone who has to deal with it later, including executors who may not know what was bought or from whom.
Metal is anonymous by nature, and paperwork is the only thing that makes a particular piece of it demonstrably somebody's property.