Last week I ordered some sandals for my wife from a Shopify store.
Checkout was the usual process until the very end when they added a post-checkout upsell offer.
These offers can help increase your revenue and order sizes but they are very dependent on the offer.
In this case it was to get a second pair of semi-expensive sandals in a different color. It wasn't something you'd impulse buy and it wasn't something that was needed (the need was being fulfilled by the original purchase).
Knowing Shopify, the offer was probably coded to just show another item from the collection. But a much better option would be to offer an accessory or product that complimented the main product (e.g. extra pair of laces).
Upsells in general? Great tactic.
But they require thought and testing to work well.
Maybe upselling a second pair was tested and converts really well for a small subset. My gut says it doesn't and this was just a default setting.
Done right you should be able to see an increase in the average order sizes with an upsell.
The Cohort Report in Repeat Customer Insights can detect this increase and let you compare it to other cohorts and other purchases in the same month (among other things).