HeyMax Miles Booster lets you earn an extra 5 MMpd on voucher purchases, for a 10% surcharge. But that's on the steep side, when cheaper options exist.
If you complain when something goes wrong, you should give credit when something goes right.
The SPG Elite Thank You Certificates are available for 2016. I believe both Gold and Platinum members are eligible for these, but do correct me if I'm wrong. These are a great way to thank any...