I figured out why I only got $21 last time. It was because they weighed me at 150 pounds and therefore only donated 690 milliliters. Evidently, 690 mL is only worth $16 to the plasma people. I didn't realize that, and it never came up before because that was the first time I dropped down to the regular rate. Up until then I had been still getting the initial, higher, first-time donor rates. The base for 690 mL is always $16, so I got $21 because of the $5 Independence Day promotion.

Luckily, I weighed in over 150 lb. today, so I donated 825 mL and made $25 ($20 for the 825 mL and $5 for the promotion). My whole mindset has changed now. Now I want to weigh more than 150 lb. so I can make the extra money. Previously, I had wanted to weigh less than that because 690 mL takes less time.