I've never been paid for mileage but haven't asked for it, either. I've been doing comp shop for awhile now and here's what works for me:
On Saturday I get the PDA, log in and get the list. I keep the PDA with me most of the week so that if I want to drop by WM and do some of the shop, I'm prepared for that. I don't do it all at one time but spread it out over the week so that I'm not in WM for hours and hours. After each shop session, I upload what I've gotten so that if something goes wrong, there's at least a partial shop. I also leave the PDA on the charger for awhile after each upload so it's always well charged.
I have a weekly goal of 73% of the shop and have been green almost every week. Last week I was red and I think it's because there were two lists and I worked only the longer one. Apparently, both lists were to be completed but the only other time that happened to me I was told that in the case of multiple lists I'm to do just the longer one. If it happens again I'll try to find out just what to do as, like other things, it seems the rules change often and with no notice. 😉
Oh, I rarely use 'item not found' as that lowers the score. I just skip what I don't find.