Get 2 twin packs of 1-Liter SodaStream Carbonating Bottles for $39.98 at HomeDepot.com. Then, add code GIFT4DAD at checkout to drop it again to $31.98. This gets you a total of four 1-liter bottles, which means each one only costs you $8. This is the best price we found anywhere (the next best is $10 each here). They're BPA free and reusable for up to 3 years. Spend $45 for free shipping, or ship to your local store for free. Sales tax is charged in most states.