A failed upsell charge is almost always an ordinary card decline from the customer's bank (e.g. insufficient funds) - not a PDQ or wallet limitation. Shop Pay is a fully supported, chargeable method for upsells; only express wallets like Apple Pay/PayPal are excluded from being shown the offer.
What happens on a failure depends on your "When payment fails" setting:
Keep In Cart For Up To X Min - the shopper can retry within that time.
Remove From Cart - the item is released immediately.
💡 The exact decline reason appears in the Shopify order timeline, not in PDQ.
