I could try again in the sun, sure.
Other than the 7-8 cm maximum on the rear of the side window, everything else is covered, and so any sunlight getting in the very rear would not affect your son's eyes unless his head was turned 90 degrees towards the window and the sun was right there. So in other words it's nearly perfect for that purpose, and better than any aftermarket blind I've seen.
About the discount....Three Point Motors in Victoria has an annual "Garage Sale" with 20 off everything in mid-October, so we went there and bought $1500 worth of stuff for our smart car and the B-Class about a month ago. Roof bars, ski rack, blinds, trunk tray, oil filters, brake pads......
I guess you would have to wait until next year?
If you are a member of the MBCA, they will give 10% off parts, accessories and service, year-round.