I've just submitted v2.3.9 of One-Page Checkout for the Zen Cart moderators' review; I'll post back here when it's available for download.

This release contains changes associated with the following GitHub issues:

#295: Correct ambiguous coupon_id on account registration (MySQL error)
#296: Correcting greedy CSS change in v2.3.6, resulting in items showing that should be blocked.
#297: Add address-fields' 'specificity' to correct browser address-autofills.