Quantities with Upsells
Upsells in general should be impulse purchases. As a result, it should be a simple one click button transaction.
With the addition of a quantity box, this adds a secondary thought mechanism to the purchaser. As a result of this mechanism, a customer could ask them self how many they want. Because of this, it has added a point of thought to the transaction. If the purchaser can’t make their mind up on how many they should get, it can be an easier path to simply leave the upsell altogether. As a result, you have lost a potential upsell.
Secondly, Mobile. With mobile now accounting for 70% of online Shopping traffic globally, space is important. When showing an upsell, the image is crucial. The title is arguably as crucial and the Add to cart button is extremely crucial. These 3 elements require space on the viewport. With the addition of a quantity box, it starts to become more cluttered.
I’ve heard what you said, I need quantity options though!
If you need quantity options, you can create a Product for Upselling which has quantities as variations. If you are tracking inventory in Shopify, this may cause an issue so consider putting a mechanism in place that tags orders with these products so you can amend quantities.