Skyrider Patrol’s triggered ability goes on the stack without a target. While that ability is resolving, you may pay {G}{U}. If you do, a second ability triggers and you pick a target creature. This is different from abilities that say “If you do . . .” in that players may cast spells and activate abilities after mana is paid but before the target receives a counter and flying.

While resolving the triggered ability of Skyrider Patrol, you can’t pay {G}{U} multiple times to give more than one +1/+1 counter.