You wont overfeed her Tegan! She is maybe going through a growth spurt? I'm sure it wont last forever
Maybe get some good cheaper snack items that will keep her going, and pile those through her.
Another thing to consider is low GI stuff so it burns slower and keeps her going longer?
Cheap items - frozen peas, toast, sultanas, vege sticks etc.... I would suggest just buying her things in the larger packets so you can give her as much as she wants without having to open multiple packets. We give jenna cheese from a block, and she loves it that way. She also loves the cheese slices etc, but if we fed her those, we wouldn't have much spare cash! So a 1kg block of cheese lasts us about 2 weeks? Sultanas can be bought in a larger packet adn given to her in a bowl rather than the little boxes. maybe save those things for when you are out and about? Same with yoghurt, maybe buy the bigger containers rather than smaller individual servings?
Drinking wise - well we only give Jenna water and cows milk, so I can't help you too much there as its not too expensive.

I dont know if you do this stuff already, but its just some suggestions. And I'm sure you can't overfeed her! Specially if its all good healthy stuff.