I think that the real issue people are having is that the way the information is provided does not look like it's giving the customer an option, but rather stating that they owe that amount. If the letter simply stated that the support period was going to run out at such-and-such date, and here are the options for renewal, then no one would complain about it at all since it's just informing the customer. By making it an invoice, the implication is that there was an agreement already made to pay that amount while the same information in a letter format would be understood as informational only. The further step of a "Past Due" invoice does not really make it clear that the service has run out, but more gives the impression that the recipient is late in making an agreed payment when really there was never an agreement to begin with. At best, this would be considered a bad sales tactic and shrugged off, but at worst I could see how it would be considered by some to be fraudulent.
The reason it could be seen as fraudulent by some is that there are some situations where it is interpreted as a bill and paid by someone at a company that may not have any idea what they are paying for, nor the authority to be the decision maker. When I had a larger cabinet shop, my wife did all the books and would not be able to tell you what Bobcad is, but likely would have recognized that she'd heard the name before and knew that it's a component of our business. She would almost certainly interpret those invoices as unpaid bills and just send a check off to pay for it even though I would not have wanted her to. In that way, it becomes somewhat fraudulent in that it appears to have the intent of bypassing the decision making process that would normally occur at a business by making it appear that the decision was already made to the people that don't make those decisions. I know that I never opened any of the mail for my business back then and would never have caught it, and I also know that my wife would have mailed the check because we just don't have unpaid or late balances. Also, the $300 or so that the support package runs is an amount that would not attract any significant attention at most businesses as it would be a pretty typical smaller expense, which again makes it feel a bit deceptive in that it is easy to slip by the decision makers because those that do the books would not see it as such a large amount that it catches their attention. If it were a $1000 invoice, it would almost certainly be questioned at most businesses. I personally had lots of little $300 invoices here and there for things like cutting tools or resharpening services, and often smaller material orders. I rarely had invoices for balances that exceeded that $300 range since in most cases they would have been paid up front, so a balance significantly greater than that would always stand out as unusual. There were probably 30 or more $100-$500 invoices (paid or balances) per month when I had my larger shop, which would make that sum fit right into normal business expenses preventing it from being questioned, especially if it's only once per year.
I have not received anything like what has been reported here, but I do feel somewhat sad that the inroads made by Bobcad the last few years seem to have just taken quite a hit with some customers in this thread that do really like the product. My hope is that these "Invoices" would be reformatted as letters in the future so that they don't feel like attempts to collect money that was not agreed to. Some sort of website system where you enter a code provided in the letter to renew the order for another year might be a convenient and effective way to allow customers to continue their support if they would like to, with polite reminders via letters or email a couple weeks before the service would lapse.