I make bank shopping for TS. I do grocery stores and inspections, with a few audits thrown in. For an inspection, I won't get in my car for less than $8 PAD if the site is more than 20 miles from me. I've asked for $17 in PAD (in essence doubling my pay) for inspections that were more than 50 miles away (and I fit one or two other MS gigs in along the way). It also helps if you remind them how inaccurate their distance estimate is when you're making the PAD request.
As for the grocery shops, wait until Wednesday before you consider taking one. When they call, let them know that Wednesday might work but you aren't sure you can squeeze them in, but Thursday might be better. If they call Thursday, they're going to offer PAD, and depending on the situation, it might be a "name your price" deal. If you're brave, hold off until Friday. If they call you Friday, they'll pay pretty much what you ask for.
It's really supply and demand. They have a supply of work that needs done. Demand what it will take for you to do it. :-)
Shopping the Ohio Valley and all around the tri-state area (Pittsburgh, PA; Parkersburg, WV and Columbus, OH.)