As a self-employed driver, you are required to submit your own tax returns for any money earned. You would only pay tax on the 'pay' part of your earning and not the cost of the products purchased. You should keep receipts for these.
The deadline for filling in your Self Assessment is January 31st when completed online. You should declare any money earned between 6th April 2017 and 5th April 2018 by 31st January 2019.
You can register for self assessment and read more using the link below:
Note: the government introduced a new tax free allowance from April 2017. This allows you to earn up to £1000 from companies like Beelivery without paying any tax. Search online for 'micro-entrepreneurs allowance'.