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STAMPS.COM Past-Due Scam Dallas Texas
I too received one of these 'Past-Due' invoices - and yes, clearly a marketing scam. It's also illegal per the "http://www.usps.com/postalinspectors/fraud/invoices.htm" page.
Do not give any of your information to them - they will aggressively ask you for it if you call - phone numbers, email, etc. Don't let them intimidate you into giving this info - it's none of their business. They are gathering all that they can so that they may bother you further - or perhaps make some more money for themselves by selling your info to other solicitous agencies. Do not click any of the links on their page - note that there is nowhere to 'log-in' (at least I could not find one) when you click "Already a Customer?" (note again that they've supplied a user name & account number for you on the "invoice")... they instead want you to click the "upgrade"... which I'll assume loads their software onto your computer (I'm not going to click it to find out however).
The link above says to notify the US Post Office - so if you'd like to see this kind of trash shut-down, take your "Past Due Invoices" to the Postmaster...