There are three different ways to offer paper upgrades and canvas prints to your clients. Those three ways include:
Upsells & Add-ons - Upsells and add-ons are specifically made to encourage people to upgrade a standard print to something that costs a little bit more, such as an upgraded paper type, an upgrade to Canvas, or even upgrading to a mounted print.
Single Image Products - Single image products are any products other than standard paper prints that require the selection of just a single image. For example, a Canvas Gallery Wrap could be sold as a Single Image Product. So could other items such as a multi-size (single-pose) school photo package, a coffee mug, or anything else that requires just a single image selection.
Multi-image & Multi-option Products - As the name implies, Multi-image & Multi-option Products are products that require your clients to either select more than one image to accompany the product, or they require the selection of a specific version (or "option") of the product. In the case of canvas prints, you might have a product titled "Canvas Gallery Wrap" that includes an option for many different canvas sizes.