The BB insurance was available when I purchased my Pre (& I declined it). The rep asked if I needed Sprint's insurance prior to reviewing my plan entirely. Once he reviewed it, he then noticed that I already had the insurance but they did offer to put it on there for me.
BB and RadioShack are authorized to sale products and make plan changes on your Sprint account. You could also always have the BB rep call up Sprint and add it to your account if your worried.
I've purchased multiple Sprint phones from BB and have had no issues with the rebate, contract, plan changes or adding insurance (when needed). They will also hand you a print out of your plan and contract after the sale is completed.
EDIT: The insurance is $7 monthly too. I believe the deductible was upped to $100 for any damage outside of wear/tear and for lost/stolen phones.