If you are going to do it, do it now!
This will allow the maximum amount of time for the miles to post to your account. If you haven't used the e-store before, make sure that you go to the retailer site via your BA account and the e-store link. Don't leave the retail site or use any coupon codes apart from the ones on the BA e-store linking page.

When I use things like this to extend the life of miles, I watch like a hawk to make sure the miles credit. I am also ready to jump in at the last minute to buy miles in case something goes wrong with the shopping transaction. I have only had to do so once.

Good luck!