Do i have to pay right away if i ordered something online and chose the "bill me later" option?

I just ordered hair extensions online and im just wondering if i have to pay right away to actually receive them. I am going to buy them but because of unexpected money issues i don't know if ill have the money right when they arrive. Will they just be shipped back or do i get to keep them and send in my money later? Do i have to pay the post office or the delivery guy before they give them to me? This might be a dumb question lol but if you have an answer pleaseee let me know