Answer:
I can put a call through to the organisers of the trip explaining that I have cash but it's not within reach and that if they reserved the spot for me, they'd get the money as soon as I can access it. Given that I am a valuable member of the student store staff, that reputation should count in times like this.
To borrow money from the cashbox would be a huge ethical risk and can easily be termed mismanagement of funds especially where the store policy does not allow such.
My value in the store lies largely on my integrity and the trust they students have invested in my person.
I'd rather miss the road trip than make a regrettable unethical decision.
Cheers!