Last time I did was 2 months ago.
Bought a CPU from a local mom and pop computer repair store
Pricematched with newegg.com (price difference was $120)
I called AmEx on Saturday 8am and the AmEx rep was slow, although patient. Asked a whole bunch of questions about what the item was, when it was bought, where it was bought, how much was the original item, how much is the cheaper item.
Then I think I had the option of waiting for them to mail me the paperwork, or I could just fax over the original receipt from the local mom & pop and the printed webpage from newegg.com. Setup the fax machine and sent it over.
7 days later, got the check from Amex.
It would be alot easier if they had an online form, or a standard paper form to fill out at the beginning.. but I guess they make it as "difficult" as possible so that people only use Amex PM, if you really, really want to pricematch.