The world of virtual currency is getting even more confusing. You can now pay via American Express on Twitter, or buy pizza with bitcoins — in both cases with major limitations.

American Express is launching a “pay by tweet” service. After linking your card to your Twitter account, you can buy selected products from the likes of Amazon and Sony, or buy an American Express gift card.

To purchase, you have to post a tweet with a hashtag referring to the product. You’ll then get a reply on Twitter from the @AmexSync account with a customized hashtag for the purchase. You then have 15 minutes to post this customized hashtag to complete the purchase.

The catch is that this doesn’t work in retail stores. All goods are instead delivered to your door within two days. In other words, there’s no convenience advantage at all: you are simply posting tweets that will make no sense to most of your followers, rather than, say, visiting Amazon and clicking a purchase button.

The only reason to buy is because some of the goods will be at a special offer price, such as $149.99 for a Kindle Fire HD tablet, down from the normal $199.99. So in effect American Express are paying you fifty bucks to post a hashtag that other people might be able to figure out as having something to do with the credit card firm.

Meanwhile a pair of programmers have come up with an idea they reckon will help promote the virtual currency bitcoins: they’ll let you spend them on pizza deliveries.

It’s a simple process. You visit the Pizza for Coins website, place your order from a restricted Dominos menu, and pay in bitcoins, adding on a 0.09 bitcoin (approx $2.34) surcharge over the usual prices. You’ll also pay a little extra as the site uses a slightly less favorable bitcoin-dollar exchange rate when calculating a price list.

The operators then take your virtual cash and, erm, use their own dollars to automatically place an online order on the Dominos website. Your pizza then turns up as usual, albeit with a delay while the operators convert your currency, meaning you may have to wait up to 80 minutes before you can eat.

(Image credit: Wikicommons user 4028mdk09 via Creative Commons license)