The Guatemalan government just did something that seems totally crazy — they told the banks they cannot accept US currency in bills of $20 or less. I leaned this when I just got back from a trip where I closed a bank account and brought back about $2000 in US $20 bills.
How this will play out is yet to be determined. In my case I talked to some coyotes (money changers). The first one offered an exchange rate of Q7 per dollar. The official rate is Q7.6. I eventually found Q7.3. Some businesses are also accepting US$20 bills but it is yet to be determined what will happen long-term. This is particularly strange as the official currency of out neighbor to the south, El Salvador, is the US dollar. In addition, any ATMs that dispense US currency do it in $20 bills.
This is mainly a generic warning to help you with trip planning. At the moment at least we will continue to accept US currency (in good condition) and you can always pay us using PayPal at the address firstname.lastname@example.org. If you book through AirBnB.com, they handle the currency conversion.
We will keep you posted but, it the mean time, if you are traveling to Guatemala with cash in US dollars, your best bet is with $50 and $100 bills which the banks will accept.