^^^ to all comments about travel expenses - it is not unusual to wait until the next following payday for travel to be reimbursed, this happens across our organisations too. I have to put in my expenses by the 3rd of the month in order for the money to be reimbursed on payday on the 24th - which means that for anything from 3rd of the previous month (or earlier) you wait up to 6 weeks to be paid. My current travel claim (to be paid on 24th July now) has expenses for all of June so far.
Fairycakes, the key word here is "reasonable". If the request to go to the far shop is just every so often, it's not unreasonable, but your manager should take the distance and the fact that you don't drive into account. We were going to be moved to an office base with a 3-hour public transport journey there and back every day - in the end that was deemed "not reasonable".
If there is nobody else to cover, then the company may have no choice than to ask you to do this journey, but since the travel is adding 50% to your working day, it might be worth asking if you can add to your Time in Lieu hours, just for this particular cover.
Otherwise - yes it's in your contract, they are within their rights to expect you to do it.