You are incorrect about the postal charges.
The charge for up to 500g is £2.95 by First Class Mail.
The reason it appears to be more is that you have not used the pulldown menu to select the less expensive shipping option. Default shipping is currently set to Special Delivery, as this is the only Royal Mail service not being disrupted by industrial action. First Class Mail, which should take 1-2 days, can currently take up to 2 weeks due to localised strike action and backlogs. When you first add items to your basket to see the prices, this may not be obvious; however, prior to checkout there is a notice which recommends the use of Special Delivery to avoid delays caused by strike action. In many cases people will gladly pay the extra £3 to make sure it arrives promptly, but we are continuing to send parcels at £2.95 just the same.
We have now changed the default shipping to First Class Mail to make the lower cost more obvious, but we are continuing to recommend Special Delivery until all postal strikes are over.
Buying one roll of film by mail order is never going to be cost-effective due to the cost of packaging and shipping, but at the prices we are offering this film I hope people will be encouraged to order multiple rolls at a time.