I haven't ever heard of that, but you do have the option of a menaje de casa, a one-time deal, that you can import everything duty free. But you have to pay a fee to have the menaje processed.
If you travel during operativo paisano times, your duty-free exemption increases to $300 per-person from the normal exemption of $50 when crossing by land. Any additional amount is taxed at 16%. BUT - unless it is clear you are trying to import goods for commercial purposes, they'll take your word on the values.
When I crossed in November, I had a nice list of all items in each box prepared. They weren't interested in it and handed me a form to fill out to list general items and current cost. Something like "used television" $50, "tools" $150. Then at the end he asked me about how much I think everything is worth in total, he subtracted $600 from that number (2 of us traveling) and assessed 16% tax on the rest. Turned out to be cheaper than if I tried to get a menaje at the consulate for $127. The nice paper I made of what each box contained did come in handy, as the aduana officer asked to see certain items and I was able to quickly find them.
On your other issue - no problem that you already filled out the form on the internet. Just fill it out again when you get to mexico. The first time I applied for a FM3, I filled out only part of the form not knowing what info to include, then they sent me home to refill it, then I had a typo, and made another form. No big deal, just make sure you bring the most current/correct paper with you to immigration.