Anyone here migrate from Sage? Sage has a concept called "prepayment" and I can't seem to figure out if there is something equivalent in NetSuite.

If you're referring to prepayments that you'd make to a vendor, there is a feature that can be enabled called "Vendor Prepayments" (see SA 100070). If you're referring to prepayments that a customer might make, there are Customer Deposits that can be done (see SA 9121)

Thanks, @Chloë. After some searching I found "Statement Charge" which ended up working for this purpose. This writes a debit into AR for an arbitrary amount which is attached to a customer and acts as a placeholder for unsettled amounts (returns, disputes, etc...)