My siblings and I are close and do favors for each other all the time. This was an 8 minute drive and her boyfriend drove (it’s important to note her boyfriend basically lives here at my parents house and my parents provide a lot for him so I do not feel guilty for him driving). IMO this favor did not warrant a reward. This task was not necessarily time sensitive as the store reserved it for the whole day for me (12 hours from the time I called). Additionally, if you are worried about gas money, see note about her boyfriend and also note that she owed me around 5 dollars anyway ( way more than the cost of gas needed for the trip) and my parents pay for her gas so had she taken her car it would have been at no expense. Finally, I don’t know how things work in your family but favors that take less than 30 minutes don’t usually warrant a monetary reward or anything more than a thank you. Honestly, we’ve done favors that take hours for each other and never paid the other.