Two Full Sets (10 Inkjet Cartridges) Canon CLI-221 with Chips $14.95 Shipped at Meritline

OK, so it's back sooner than expected and at the lowest price that I've seen them offer by $5:

Canon CLI-221 with Chip (CLI221, PGI-220 BK, CLI-221 BK, CLI-221 C, CLI-221 M, CLI-221 Y) Compatible Inkjet Cartridge Combo with New Chip

One Each of PGI220 Black, CLI221 Black, Cyan, Magenta, Yellow

Regularly $29.95, now $14.95 shipped with code MLC125272090950NL1. The code has a limited number of uses so don't wait to order if you really want this.

Compatible with:
Canon PIXMA iP3600
Canon PIXMA iP4600
Canon PIXMA iP4700
Canon PIXMA MP560
Canon PIXMA MP620
Canon PIXMA MP620B
Canon PIXMA MP640
Canon PIXMA MP980
Canon PIXMA MP990
Canon PIXMA MX860 (also said to work with the MX870)

To those of you who own a MX860, I have a question: I run through these inks very quickly. The fastest to run out is the 220, even though it has more ink than the other cartridges. Perhaps it is because I also print a lot in black. What puzzles me is that for every 5- (or even 6- or 7-) sets of the colored inks, I use up only 1 black 221. As a result, I have a whole stack of them. I don't understand why this happens, and how can I avoid it. Perhaps make "black" in "color", so the 221 is used instead of the 220?
Any help will be appreciated. Thanks.