Well presumably he gets paid at the normal active duty rate when he’s called up, or whatever the UK equivalent is.

When you join the reserves you have to go through basic infantry training (you can't just go straight to Sandhurst like if you went into the regulars). You're paid £40.13 per day at the moment (I think I was paid something like £30 a day when I went through it a decade ago). I don't think anyone goes in for the money - I've made a days training sat on the toilet after lunch at my last contract gig.

http://www.armedforces.co.uk/armypayscales.php#.YksfMKTTXDs the salary for an actively-serving Major ranges from roughly £55k-£65k pa. The author is probably earning at least twice that at Shopify, if not considerably more.