Greetings Debi,
If you look at the link below, Amazon is explaining what qualifies as meltable along with other info (time periods that meltable products can be moved around, temperatures etc). It also indicates that if you consider your product not to be affected by the heat/storage duration you should mention that accompanied by evidence (I guess something like a shelf-life analysis in a laboratory under the specified conditions).
So the letter should look like a Letter of Conformity (or CoA or a Technical Data Sheet/Description of your product whatever term you are familiar with) in which you mention that your product is stable under said conditions (=not meltable) or if it is meltable but your experience says otherwise then attach a shelf-life analysis (best solution I can think of).
https://sellercentra...rnal/G202125070
Hope this helps,
Regards!