Nexus 4 payment question

Currently I am planning to use straight talk, and with that you need to purchase a phone full price. Seeing as many phones full price are very expensive, the launch of the Nexus 4 was a great surprise with its reasonable price and being sold unlocked. My question is, due to Google's terrible stock of the phone the 16GB is pushed back to 5 to 6 weeks. If i bought one now would they charge me and make me wait 5 to 6 weeks until i get the phone? Or would they charge me when the phone ships?