Unfortunately they seem to charge whatever they want to on the day. Either the rules are so complicated that they don't understand them, or they do but they're banking on us not knowing. I've never paid the same rate twice, for very similar gear. And I've always been charged duty on educational books (PADI manuals) even though they were officially free of duty. I appealed once, and was told that I didn't have to pay if I didn't want to - but of course I wouldn't get the materials either. I have been offered "cut price" clearance, where of course the money goes straight into someone's pocket, but I've never fancied that.
I once tried to import a single fairly high value item directly, without using a broker. They first refused to deal with me, then they stone-walled and procrastinated. When I printed the relevant rules from the web and showed them to the Customs people at the International Airport they finally relented and accepted the paperwork I'd offered them initially - but after they'd made me go there three times and to their head office at the seaport twice. I was temporarily importing exotic dive gear for someone who was visiting - the gear was actually cleared almost three weeks after it arrived in Belize and several days after the owner had departed, angrily blaming me for the hiatus. Cost me a fortune that of course I couldn't recover.
Most people I know who import on a regular basis have some sort of smuggling scam running. That can also prevent the pilfering that seems to be normal in the Port of Belize - never have I received everything listed on the shipment.
But for an AED it makes most sense to apply to ACER - they will import it for you and charge you a very fair price. And they know which models are much better than others. Ask "ditchdocdiver" at Sueno.