While I would expect this to happen, in law its a bit different
The contract is not formed until: - their offer, your acceptance of said offer AND finally the bit that always is curiously missed, their final confirmation back of that acceptance are all completed.
Most online firms now have a clause in the T&C's which states that the contract is formed at dispatch of the said goods - which I suppose they reckon the be the point when they effectively loose control of them so are deemed to have accepted the terms.
(Similar I suppose to when checkout staff hand you the goods in a shop)
To be honest I've always regarded the confirmation email back which usually details the exact pricing of your purchase as the final order acceptance part though I've never had to test it - there is probably an exclusion clause somewhere to the effect that such emails are not order acceptances.
This is how the online firms quite legally extract themselves from the various mispricing disasters where a £500 telly is on the web for £5. It helps of course if they only charge the credit card at the point of dispatch otherwise charging a CCD at the point of purchase might be regarded as the final order acceptance. (lets gloss over here the difference between a charge being charged and a reserve authorisation being put on a card in advance of a charge to be applied later)
I did once have a similar situation on a multi month long lead time item from a specialist supplier (non computing) and made a point of checking with a view to a forthcoming general price rise they had announced that the price I paid would indeed be the price ruling at the time of purchase and not the prevailing price at the point of delivery when my card would be charged. (Couldn't be arrsed to read the T&C's !).