putting this out there before anyone else winds up in our situation.
the issue: we just print off customs forms as needed... i think we checked them a few times in the beginning several years ago, but it was fairly boilerplate commercial invoicing, etc. i am going to assume that something changed and we arent the first to find this... but the shipping amounts and therefore total are wrong on those forms.
the problem: shopify is using the shipping COSTS (the price you the vendor/manufacturer pay) to fill out the customs forms... and if you are like us, where your shipping costs are marked up (or... not really marked up - just not discounted all the way down to our cost) to accommodate handling, packaging, forms, etc - those costs are not reflected on the customs paperwork.
if you are offering free shipping on all your products - then it will be just fine. if you arent... and have markup of any kind - read on.
for most - this discrepancy is relatively minor and results in a dollar or two difference. for us and anyone like us... our handling/packing is extensive due to the product being shipped, size, weight, forms needed (some can be wireless transmission devices), as well as all the hoops we have to go through to process them internationally. the discrepancy can be upwards of $60-150 in shipping charges on a single order.
if you ship enough internationally... and get enough complaints or flags with a few dozen packages coming in every day - it starts creating a massive issue with those internal customs departments.
the only solution we can find is to alter the product cost as shopify first insisted that this was not their issue... but then changed course and admitted the problem, but their solution is to "use a third party app" (which will not work for postal shipments). but that would be catastrophic for our business as it would make us look more expensive than any other competitor (and as it sits now... the grand total after shipping, we are the cheapest)
exploring other options currently... but this is going to take a massive rework of our 20k+ products, and simply put... the benefits of shopify are starting to seem not worth it.