Took 5 HP and HP-compatible cartridges (15 & 17) to Staples this week.
Their current offer here in the Sacramento area is $3 "credit" which their
corporate office pays out to you every 90 days. And you can only turn in 3
cartridges per day.

I will be calling Office Depot, as I've taken them there before and got
*something* on the spot (paper, I think?). But that's been a few years ago,
though also in the Sacramento area.