Questions about cancelling National Postal Mail Handlers Union dues

Hey everyone, I was recently laid off amidst all of the lovely government "restructuring", and during the chaos I forgot that my previous enrollment in MHBP meant that I was also enrolled in the NPMHU. I just received my bill for annual dues, and obviously I would like to stop being billed from here on out since I no longer have MHBP health insurance.

As far as I can tell, I need to complete PS Form 1188 within 10 - 14 days before my annual enrollment date (which I'm sure has since passed, so I'll have to do it next year) and mail it to the NPMHU national office.

My questions are: Are these the correct steps to take? Does it make a difference that I'm no longer a federal employee? Is my annual enrollment date the same as the billing date on the bill sent to me? Would it matter at this point if I just didn't pay the current bill?

Any help would be appreciated!