I am not a lawyer, cop or customs officer; I am not from Canada. But I think I can still help you.
The customs found it suspicious that someone bothered to send a 6$ worth item to you from another continent. But luckily, what you bought is an underground gray-market gaming device; not an item of apparent high value like a DS console, mobile phone, iPod or the like. Considering this, you should be able to get away with stating it really is worth 6 dollars. You can actually buy an SD card reader from some sites for 2 or 3 dollars, free shipping. I don't think the customs will run an extensive online search on R4 (just to eventually find out it's a harmless object worth 30 or 40 bucks). So my suggestion is you send that fax confirming the declared value. Your other option is to state that the seller tried to play it smart but you're honest and admit the R4 is worth more than that. Nothing bad will happen to you, but you're likely to pay a relatively small customs fee.
By the way: screw strict customs! We're not wholesalers, FFS!Â