yes that works.
I enter my NW number in the page where you log into the mall, and print that page--which shows how many miles you are supposed to get.

I save any emails about any promos and print those in case email crashes, etc.

I save the page that confirms my order with whatever store I used. I print that too.

I print the email that confirms it is placed and the one that says it shipped.

I print the statement showing whatever transaction took place on whatever credit card I used related to this purchase.

I keep it all together in a folder marked with what I planned to receive in terms of miles.

If it comes, I toss the pages or maybe hold onto them in case a like-deal or promo is still going on.

The theory, which works, is that if you have to present this sort of evidence to the airline, it may be a pain and it may take a while to resolve but the guy with the paperwork wins.

I just went thru one that took 3 months but resulted in both the miles that were missing and some mileage-compensation for having to go thru all of that.

I would rather their mall portals just worked without hassle, but this has not always been the case. Nevertheless, I do find the malls to be among the better ways to earn miles on airlines.

They ought to have a line in there which says (and I am actually quoting from someone AT NWA who eventually credited me missing miles) "Most of the time this works without error and the majority of our customers will receive mall purchase miles without issue..."

If you need to see more on that, visit some threads on missing miles in the NWA forum.
MM

PS Screen shots and printouts do not lie. But if you just save a link, that site or page could be taken down so you lose your 'evidence.'