Seems quite a bargain. However, as it is a preserved railway, might just pay the top whack.
No, use your discount. You can always return again a few times so the heritage line will get its money in
regular instalments so to speak. Also, you could take a friend or put some money through the till in the shop.
In general when I am out & about I try to use the local pub, post office (even just to buy some stamps), bank, and shops ~ spread the love.
Just an idea ~ it's of course up to you......