Do you have to have certain pastlife karma to use a computer?
Karma is a complex dhamma that only Buddha saw it thru and thru..   However, Buddha taught us 6 things to know about the fundamentals of karma

For sure, Karma is not a fate.  In Angulimala Sutta, a Sutta about a misguided serial killer who would have committed matricide if Buddha didn't interfere with his killing mission.    

You can give $20 to someone but then you will never know if it was his good karma that let him run into you?  or his good karma that you have money to share.   Karma is a complex thing that is imponderable or incomprehensible, Acinteyya (Pali).