Why are orders showing as partially paid?

Under certain conditions, a payment may not be collected for a post purchase promotion resulting in a "Partially paid" financial status. 

1. A customer used a debit card on the original order, redeemed a native post purchase promotion, and did not have a sufficient balance remaining on the card resulting in payment to fail on the promotion.
2. A customer redeems a post purchase promotion on the thank you page and fails to re-enter payment details.

Both of the above two scenarios will result in no payment being collected for the promotional products which causes Shopify to change the financial status to "Partially paid" since payment is missing from part of the order:

There are four options to resolve the outstanding balance:

1. Remind the customer to pay the outstanding balance due.
2. Remove the unpaid upsell product
3. Restrict the promotion to the "Native post purchase page" location which supports 1-click payments and won't show for orders placed with payment methods that don't support 1-click. Note that ~1% of orders will still result in a "partially paid" financial status due to failing debit cards from scenario #1 above.
4. For future orders, enable the "Auto remove product when payment is missing" under Settings > General > Auto remove: