One suggestion would be to buy a Two Together railcard: this would save 34% off the total. The card is £30 but you may be able to get it for £27 online with the promotional code "ONLINE". But ordering online may be tight in terms of the railcard arriving through the post by the 27th. The card would just about pay for itself on this one trip, so it's only worth getting if the OP will be making more offpeak train journeys with the same travelling partner in the coming 12 months.
EDIT: without a railcard, the following is a way of reducing the price but at the cost of increased journey length (prices stated are for two adults):
Preston-Liverpool: Northern Duo £19.80
Liverpool-Chester (via Birkenhead): £13.80
Chester-Shrewsbury: £21.60
FURTHER EDIT: without a railcard, for 2 adults returning the same day:
Preston-Frodsham £25.40
Frodsham-Helsby £7.40
Helsby-Shrewsbury £21.60
Total £54.40
Outward travel on the 0958 arriving 1227 and changing at Warrington Bank Quay and Chester. Various options for return travel, but in either direction journey must include a train between Chester and WBQ that calls at both Helsby and Frodsham.