amazon sales tax question

as per google ” Keep in mind that Amazon charges 2.9% of each transaction in order to collect US sales tax. “

My question is follows:

does this start from the very first sales transaction or after u meet a threshold or number of transactions?

also can someone explain to me how the 2.9% works because most states have sales tax 6% or 7% etc…so why does amazon only charge 2.9?

thanks for your help