Well, if you order in the local webshop and ask for collecting in-store and do it very well in advance, it might work. But I'd call the store and ask them if they are OK with it and would keep it for you for many weeks without sending back or something.
It can be around 8 weeks, sometimes it can go down to 3, but they say 'up to 12 weeks' on the site for a reason, it really can be 3 months. So if you order it in May, it can be ready before September for sure, but the store should store it for you for quite some weeks, depending on the manufacturing time. But since you pay in advance, they should be fine with it. The only thing is if you have any problems with the product, dealing with the issue there or waiting for an exchange can be tricky.