For instance, my transaction search is to find all customers that purchased a sub item of X. So I used the Group summary type on "Name". But some customers purchased the sub item of X one time, and some purchased it 50 times. I only want to send the email to them once. But I'm not 100% sure how many emails they'd get

Yeah I don't know the answer, I would test it with a fake internal email (or fake customers if you already got those) and just delete the transactions afterwards.

Agreed, that sounds like the safest option

Yeah. it definitely sends many times

@ericbirdsall rebuild the selection search as a Customer search with joined transaction records to get your selection. Your WF would be a Customer WF

@Netsuite Tragic I don't think I can get the information that way. I'd need a "double join" of "Transaction fields..." then "Item fields..." to get the "Sub Item Of" field. I think if I was only looking for customers that bought a specific item (or even a few) I could use "Transaction fields..." with "Item" and list the item or items I was looking for. But it's over 10,000 items that I'm looking at. So I believe I'd need to add each item one at a time to that list. Please correct me if I'm wrong here

How about an email tracking checkbox on the Customer which gets set upon sending of the first email in this campaign. When the WF checks the next Invoice, then the checkbox is already set and wont send. Or two WF, one transaction WF withthe big search to set a Customer flag for sending, the next a Customer WF to send to all the flagged customers.

I ended up going the route I mentioned above. I exported the summarize transaction search with the Customer Internal IDs grouped, then created a "Send Marketing Email" checkbox. Then reimported the list and marked "Send Marketing Email" to true for all those customers. Then used a Customer WF to send the email (and to set the "Sent Marketing Email" back to false) This worked for me