I am a mileage plus member and booking a trip to Germany on a Lufthansa flight. I hate that if I book through United I can't select my seat when booking so am considering just booking on Lufthansa with my mileage plus frequent flier number. Is there any disadvantage to this from a miles earning perspective? Is anyone aware of a way to book through United and then go back to Lufthansa to pay their fee for a seat assignment? It doesn't appear That I can select a seat through United even if I book premium economy which seems odd....

thanks in advance for any help here!