OK. So you can't tell the gender of the owner by the possessive pronoun. It must match the gender of the object that is owned. So if you give a pair of matched towels as a wedding gift, it doesn't make sense to have them embroidered "his" and "hers" ("suo" and "sua")? Is everything in Italy unisex?
As you said, the gender of the pronoun doesn't match the owner but the object itself. So, towel in Italian is l'asciugamano (masculine): you can't use "sua", you would have to write "suo" on both and you wouldn't distinguish them. Most likely, you would use "per lui" and "per lei" (for him, for her).
This is American!!!! take great care because in English it means "her KNICKERS". THE only USES OF PANTS IN English(NOT American) are a way of breathing i.e the dog pants, or as a short form of the word Underpants I.e. small clothes or underwear , knickers!! Knickers!! can be used as a childish expletive as in "Knickers! I've left my homework behind" although probably many older kids would just use F**k!