It has been awhile since this happened to me, but I believe your insurance coverage should be "transferred" to OWCP and not "terminated" by the employer, or perhaps both are to take place, as one terminates and the other takes over, but you should receive clear instructions as to what is taking place.
I don't recall exactly how mine went down, but at first I started to receive invoices from Eagan, MN for my portion of the insurance premuims and suspected something was amiss. I did pay a few invoices to insure coverage, but also contacted my Senator, who made an inquiry on my behalf and the situation was rectified and my insurance coverage was transferred to OWCP. It took an additonal 9 months for the employer to reimburse the payments I submitted.
Also be aware that the premiums will now increase to the OPM rate paid by other Federal employees and not the Postal rate you pay now. You are to be provided refunds of these overpayments from the Post Office every 3 months, which could amount to approximately $100 per month depending on your coverage. You should receive your first refund automatically after about 6 months and if you don't ask here about it. See chapter 4 in this handbook and you may want to investigate exactly what is going on with your "termination" and insure it has been "transferred" to OWCP and you may also find that information this handbook.
http://nalc.org/depart/cau/pdf/manuals/el505fdr/505url.pdf