payment period; term of payment
Business term referring to the agreed timeframe for settling a payment, such as net 30 days. The サイト part comes from 'site' in the sense of a scheduled date or period.
See also: サイト
The payment term for this transaction is end-of-month closing with payment due at the end of the following month.
Could you extend the payment period to 60 days?
Compound of 支払い (payment) and サイト, from English 'site' used in business contexts to mean a scheduled date or period. The exact derivation of this sense of サイト is uncertain, but it is established in Japanese business terminology.