The Product Add-Ons plugin by WooCommerce is a great plugin for upselling opportunities. With this plugin, you can customize the checkout field or add options/add-ons per product so customers can customize the products however they want. If a customer wants to change the fabric, add a special text, or get gift wrapping, they can easily do this by the different product add-on options available.
You can charge a fee for each add-on in a flat rate, quantity-based, or percentage-based format. You can show the add-ons in different selection fields such as text input, buttons, etc. It is one of the best WooCommerce upsell plugins available.