Alpacas do it sitting down?

They certainly do. The male begins to "orgle"- a sound unique to an alpaca and is a cross between a gentle roar and a yawn. With great orgling and flaring of nostirls and popping of eyes the male chases the female around and then placing his forelegs over her back forces her to the ground, where she will sit (kush) in the mating position, very calm and regal, like some old victorian aunt.

The male then sits behind her and gently orgles away as he makes his deposit whilst she responds in a satisfied manner in a stately and dignified way as befits an animal of such beauty. Alpacas are dribble ejaculators which means that this sedate mating session can take anything from twenty to forty minutes.  

Of course should madam be pregnant she will normally spit him off in a fashion that is far from regal and normally results in her suitor being covered in small dollops of half chewed and very smelly cud.  

A male alpaca (Macho) is ready to perform at two and a half to three years. A female alpaca (Hembra) normally matures and is ready for mating at 18months, a factor which is also dependent on body weight.

A female alpaca will normally be receptive to mating ten days after giving birth. The gestation period for an alpaca is eleven and a half months. Alpacas lifespan is between fifteen and twenty years and one would expect a female to be fertile to at least twelve years.