The trial enables shoppers to ask, at the point of order, for items to be delivered in the evening between 6pm and 10pm.
Royal Mail said it was in talks with other retailers about them using the evening delivery service.
It forms part of efforts to cater for people needing delivery of items they have purchased at times more convenient for them.
It has pledged to extend the opening times of more than 600 of its busiest delivery offices until 8pm on a Wednesday.
Recently a survey by Ebay of more than 600 online businesses threw up the finding that 84% of them wanted Royal Mail's service standards improved.
The top five things they said needed to be improved were notification of the time or day of delivery; evening deliveries; evening collection times at postal depots; better weekend deliveries and evening opening times at post offices.
The move comes as Royal Mail's future is being debated by politicians as part of the progress of the Postal Services Bill through Parliament.
Business secretary Vince Cable is in favour of modernisation in part by selling a stake in the state-owned company to a private company.
Royal Mail is also hoping to secure more cash through operations, having applied to Postcomm last month to be allowed to earn an extra £100m by raising prices for large business customers.