Code worked for me (as of 10:55 am PST)...
I had to select in-store pickup in order for my order (chair and floormat) to complete successfully after getting the additional payment error message. FWICT--in my particular case the order was separated into two separate orders; apparently, the floormat had an earlier delivery date than the chair, so I guess the online ordering system--having caught this--separated the two items into two different orders, each w/ its' own order number. Given that the chair is $39.99, the application of the $40-off coupon resulted in a negative price amount (-$0.01), thus generating the error message...which is kinda backwards in its meaning (as applied to this particular order) since you're not really "short" on payment...unless one can also interpret "additional tender" as adding enough items in order to make the order total a non-negative one.
Hopefully they won't ask me for the actual coupon; worse case scenario is that I'm out gas money and 20 minutes of my time...LOL!!!!