nope Blue Iris won't use my serial number I just paid for, all the while saying it has already been added to my account.
It is a little confusing, but what you bought for < $40 was an extended support key, not a Blue Iris license. When "activating" Blue Iris you must still use your original license key, then to use the extended support key you enter it in Blue Iris Settings > Upgrade/renew. That applies the additional support time to your original license.
If BI rejects your original key saying too many activations, then you need to contact BI support to have it reset.