Hi Crankyusi,
I transit Guam often and have family and friends there, saying this I have never had a problem with customs or taxation issues.
The way I understood the use tax is that it is a tax charged if you live in Guam and order in items (delivered via vessel / mail / luggage) and you are using the items yourself. Not to be applied to people bringing in items into Guam for other use, and the limit is $100.00 USD.
" Use Tax: All personal property imported into Guam for use or consumption by the person importing it is subject to four percent (4%) tax on the landed cost. Property imported for resale is not subject to this tax.
The tax does not apply to property having had a substantial use by the person bringing it into Guam before importation, or to machinery and other moved when the contract or other work is completed.
No tax is imposed when the total value of all property imported by any one person in any one calendar month is $100 or less.
Purchase of personal property on Guam for use or consumption from a person not liable for Gross Receipts Tax is also subject to the four percent (4%) use tax. The $100 exemption also applies to such purchases. "
Being a person bringing items into the territory, not for your use, and a gift, then you shouldn't have to pay anything and you should have no problems at all. Not living there, and only staying a day or two in Guam, I've never been asked if I am leaving items here, bringing items in for other people, etc...