As someone who has been using it extensively for months, allow me to explain the mechanic. I have Double Pay 1, meaning 1 hit is all I get. You must kill a mob with the last hit you use, and multi-hit skills are prone to “killing the mob after all hits” even if the first hit should kill it. A bug, but one you have to just work around. So it’s best to use a single hit that is guaranteed to kill something. I generally weaken then enemy, activate Double Pay, and then Spear Lunge to finish them off.
When you successfully kill an enemy with Double Pay, it will ALWAYS give you 2x experience. If the enemy drops any items, those items will be doubled, as well as all silver it would have dropped. Double Pay ceases to do anything until it’s next activated. If the enemy drops silver but no items, it will drop double silver. Double Pay ceases to do anything until it’s next activated. If the enemy drops ABSOLUTELY NOTHING, Double Pay gives you your 2x experience and then goes into what I like to call “hibernation mode.”
This is my favorite part of this skill. While in hibernation mode, Double Pay will not track your number of hits or way you kill enemies or anything. What it WILL DO is carry over its double-silver-and-or-item effect until the next kill you make which drops silver or items. I have successfully Double Pay’d a mob who dropped nothing, and then killed 10 mobs in a row who dropped nothing until #11 dropped double its items and silver. Hopefully that explains any troubles you’ve been encountering with the skill.