An order bump is typically an optional add-on presented near checkout; an upsell is a follow-up offer presented after or around the initial purchase.
Relevance matters
The added offer should make the original purchase more useful, faster or more complete.
Keep the choice clear
Avoid preselected or confusing additions that make buyers unsure what they are paying for.
Watch the customer experience
Higher average order value is not useful if refunds, support burden or distrust rise.
Measure incrementally
Track attach rate, revenue, margin and downstream satisfaction separately from the core conversion.