I believe Paypal didn't run this promo thru shopping portals.

I think the question should be whether the merchants, which you have your eyes on, are currently participating in this Paypal promo (the most updated list can be found at https://www.paypal-promo.com/holiday/slider/index.html). Or you can simply visit the merchants' sites and see if the big Paypal 20% Promo banner are flashing under your eyes. Paypal does not even provide a comprehensive list. Remember "currently" participating is very "dynamic"/illusive since it can change in ANY minute, hour or day. All I know is the list is shrinking.

After making sure the merchants are offering this promo, you can then use the airline portals (if the merchants happen to be in those portals) and get the bonus miles. Paypal didn't make it easy for us to get the $50 max. cashback ($50 is the capped amount). So, during the promo period, if you are spending more than $250 ($250 X 20%=$50) in all the participating merchants, there will be no need to deliberately use Paypal Checkout any more.