they should
The real answer needs to come from either the merchant or the network you get the offers from. But it is not uncommon for one customer to purchase more than one product, and you as an affiliate should be compensated for all purchases.
However, because of abuse and fraud, and depending on the nature of the offers, merchants and advertisers will sometimes limit transactions by the IP address or credit card number etc.. so they don't get a one time purchase processed and paid out several different times.
Hope that helps...
