Post-purchase upsell offers will only be shown to customers paying with a “direct” Credit Card processor available for Shopify Checkout in your region or via the Shop Pay or PayPal Express digital wallet payment methods.
For a complete list of supported credit card processors for your store and region, click into the Settings > Payments > Third-party providers section of your Shopify admin and then click on the “Choose third-party provider” button to see the list.