Allowances into Guatemala in my personal luggage exempt from duty

Hi, Can anyone share their knowledge regarding what visitors may bring into the country for their personal use that is exempt from duty? We travel to sail on the Rio. Our experiences have been so variable that  something concrete would be most helpful. We last paid $250 US to a group of customs folks, then later found out that we should have asked for the $$ value of personal exemption.
Is there a SAT link that  details their current policy?
If you enter by air, are you entitled to bring up to US$300 worth of goods in addition to the goods included in your personal luggage, and that you are allowed to combine this amount with family members?

Many thanks

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