@Mana I have a $90/90-day plan, I used a $100 Payment Voucher when joining PM, deposited another $100 Payment Voucher before it was time to renew, activated Autopay the next time around. Sometimes an unspent amount sitting in my self-serve balance, sometimes exactly zero, always another $20 or so from Community Rewards, Referral Rewards, and Autopay Rewards.
I've renewed three times now, the last time with Autopay. I've set up several friends who started with Payment Vouchers or credit cards and renewed with Autopay. Payments were always processed as described in my above post. Not a deliberate or methodical experiment, but enough for me to observe how it works.
Having assisted some referred friends, I've learned that (under normal circumstances) the PM mods can always correct minor billing issues - although it may take a day or so for full two-way communication of the issues, then some hours for their changes to filter through the system. And I've learned that billing just keeps on going indefinitely until something changes - most often it is the user who changes something, very rarely when PM changes something it causes a bug.
