On one of my "drive 4 hours, pick up parts in Calais ME, re-enter Canada and pay 5% tax, and then drive home 4 hours" trips, I made a back of the envelope calculation error on the value by $1200, estimating About $2.1K rather than the correct $3.3K. Now, I have always stopped and stated, "I have a lot of antique car parts with me as a personal, non-commercial importation, all the receipts with me, and I have to pay tax on it all" It was over 90 that day, and the A/C was not working again in the Jeep, so I was a bit stressed. When they found the error, it was like WWIII had started. It had made their day. Fourth degree grilling, even though I had all the boxes open with receipts attached to each, and another copy in hand, and the envelope where I had goofed on adding up about 50 separate little items. I have never tried to evade anything, and keep receipts for $2 items I might purchase locally at the hardware store, for example. Still, they threatened me with seizure of my car and huge fines "for not reporting the correct amount of imports". Apparently, you HAVE to make a $ amount declaration, and they can use that amount you guesstimate to assign penalties if you are wrong!!!
Now, I just state that "I have all the receipts, I don't have a calculator with me and I'm hopeless at adding things up in my head, and I need to have someone add them all up properly so that I can pay the tax owing. There is enough stuff here that I have to pay tax, but I don't have any idea how much, because I paid for different items over time and today I'm just bringing in the items that have arrived to date" They try to get me to indicate a $ amount, but I just keep smiling and say "I have no idea at all, but I have the receipts and somebody with a calculator can figure out how much tax I have to pay." They hate that, and but eventually give in. Sheesh!!!!
The only thing that is a consolation to me, is that US Customs is much more picky.