Think I finally figured out how to squeeze out maximum shekels from the game. Still have no fucking idea how the metagame is supposed to be played, but that's unimportant. That's for people who join corgi because they think they're going to win a free game and post about it on Reddit. Capitalist swine such as myself instead deign to occasionally poke at the damn thing until repeatable results fall out.
Step 1: The red number listed for your odometer is your daily "Tank" for collecting tokens. You aren't exchanging them yet, but you will be. The number you start at varies (Wildly) from person to person, but you're wanting it to reach 15,000 because that's the magic number necessary to spit out a $5 coupon. How do you raise the number? Buying things, unfortunately. It will increase by 100 per $1 spent, so that will give you an idea of how much you need to spend to reach your goal. Your odometer resets every day (As in, if your maximum was 5,000, you reached that, you "Spent" that, you couldn't climb back to 5,000 until the next day)
Step 2: Once your odometer can reach 15,000, you need to scroll down to the "PLAY ANY GAME WITH STEAM ACHIEVEMENTS" area. It'll either list games you've played recently with Steam achievements, or show none. You need approximately an hour of played time in your recent history for a game to show up in that list, so you need to either play or just launch the program and let it sit for an hour. The amount of points it's worth is based on achievements unlocked, so ideally pick games you've played at least some (Haven't yet experimented with cheating achievements since honestly it's been a moot point for me, finding games worth over 15,000 points has been easy). You're ideally wanting a game with 15,000 points or more, excess will be lost but it's no big deal since you can just idle new games in there.
Step 3: Once you have your points in the odometer, click boost. It appears that boost will convert your points into tokens (What you actually need for the coupon) at a 1 to 1 ratio in increments of up to 1,000. Keep at it until you've got your 15,000 tokens.
Step 4: Get your $5 coupon
Step 5: ???
Step 6: Profit
There are still some unknowns. First, I'm not sure if multiple $5 coupons can stack. Second, I'm not sure if you can unlock more than one $5 coupon (Although I asked someone who said you could, haven't yet confirmed it myself). Still, even if the $5 coupons are one-per-order, this could theoretically mean after an initial chunk of money to reach the magic 15,000 odometer window you could idle a game you've already played with achievements once a day for one $5 coupon each day. Also no idea if there's a minimum price for using the coupon, so maybe you could use it to get some DLC for free or whatever. Seems to encourage making your purchases in drips and drabs over the sale to maximize your coupon use however. This isn't completely set in stone but I have finally figured out how some of it works by fucking around with it off an on this evening.
Also the end result of all that rambling means I would need to spend $120 to get my odometer in the range to get a $5 coupon every day. So if they DO stack that would mean approximately $85 in store credit, which would make it a pretty decent deal if there was $200 of stuff I wanted (Basically buy $120 of stuff during the sale then get a free preorder of a new game too). The coupons stacking is a fairly large if, though. This concludes Cow's autism corner.
Edit: Actually doesn't conclude it yet since I want to reiterate, I still don't have 100% confirmation that you can even get multiple $5 coupons in the first place. Only people who would know would be people who dumped $300+ on the first day or had a freakishly huge odometer range for some other reason.
Edit 2: Cheating achievements works fine. Just took Primordia from a 6,543 point game to a 22,308 game, but in my defense my achievements were fucked on it anyway since I played it ages ago in offline mode when offline mode was still spotty for achievement tracking.