You will need to work out a rate that suits you, so you make a touch and pay for your outlay,
Insurance,PPE, Application eqpt and not to mention recouping the money you paid for your training.
The amount of flasks you require for the job around £20 + Vat each, its not worth making a mark up on the price per flask with most farmers, they are not stupid and can buy it as readilly as you (without a ticket), but i would charge for your time and fuel collecting it.
An hourly rate that suits you and takes in the above, so your not doing yourself out of pocket.
Happy days.