I agree with the previous comments. I will accept for the moment that it is a system error. When the amount was reduced from $35 to $5, I would have politely told them that I was willing to do the shop for $35, but not for $5. That would almost certainly be true for me. If it took $35 to entice me to do the shop, then $5 is not enough.
Airlines sometimes post extremely low fares by mistake. You can Google to find past cases. For instance, maybe a flight from San Francisco to London is normally $700 and it gets posted for $7.00 due to human error. Before the mistake is discovered by the airline, 350 people book the low fare. What happens next varies from airline to airline. Some honor the ridiculously low fare, benefiting from the goodwill and all of the media buzz that the $7.00 fare has generated. Some airlines just say, "Sorry. It will cost you another $693 or we will allow you to cancel without any penalty."
So, in my view, the MSC has two reasonable courses of action which parallel the airlines. 1) Honor the $35 bonus because one of their humans or their system caused the problem. 2) Allow you to cancel without any penalty. Both are IMHO honorable approaches, although as a shopper, I know which one I prefer.
Shopping Southeast Pennsylvania, Delaware above the canal, and South Jersey since 2008