To help prevent conversion duplication and support value-based bidding, which unique purchase identifier should be shared?

Transaction IDs should be shared. A transaction ID is a unique identifier for a single order, and passing it with the conversion lets Google Ads recognise when the same purchase is reported more than once — a common occurrence when someone reloads the confirmation page, returns to it from their email, or when both a website tag and an offline import cover the same sale. Without it, one order can be counted several times, inflating reported conversions and conversion value. That matters beyond tidy reporting: Smart Bidding treats inflated values as genuine, so it will over-invest in whatever produced the duplicated conversions. Transaction IDs are also the key that makes offline conversion imports and adjustments work properly, allowing a later refund or cancellation to be matched back to the original order and its value corrected.