Jun 11, 2019 - 06:08 PM
The best reviewed one (4.7 out of 5 stars, 1200+ reviews) that allows true, post-purchase 1 click upsells appears to be Bold Upsell:
There is another one called "Discounted Upsells" with a higher review rating (4.8) more than twice as many reviews (2,600+) but I do not believe it allows true, post-purchase 1 click upsells.
Most people do not seem to understand the difference between recommending other related products on the checkout page and true, post-purchase 1 click upsells, where the customer does not need to re-enter their credit card.
The latter usually have a higher take rate and don't risk 'killing the sale'. I explain the difference in this post:
Bold upsell also has a neat "Upgrade" feature that replaces the base SKU with an upgraded more expensive (and likely more profitable) one. Depending on your product line-up this can boost sales considerably. After all, It is always easier to get someone to upgrade once they have made a decision to buy, than to sell to a new customer!
If you don't have a premium SKU you can always prompt the customer to upgrade to a "bundle" rather than a one time product.