in 6 months - payment is due in 6 months time. But it is not very specific when its due. within 6 months - payment must be made in the coming 6months, but (depending on the circumstances) it might be expected that payment is made before then if possible. In this phrasing payment after 6 months 1 day is clearly not acceptable. upon - implies a specific moment. after - not necessarily a specific moment, any time following an event. So if this were the terms and conditions of a purchase I think a lawyer would write: "upon receipt ... within 6 months" as this would be more specific - which is important if the matter becomes legal. "in 6 months ... after " - is more open to interpretation.

Payment in 6 months upon receipt of the goods. There are a few ways to interpret this sentence. For example: You must pay when the goods arrive, 6 months from now. But, it can also be interpreted like the sentence below. However, because it has a few ways to interpret it, the one below is better. Payment within 6 months after receipt of the goods. **This can be interpreted as: after you receive the goods, you have a total of 6 months to pay for them.

@Sean Thank you:）

The payment will be made within 30 days after receipt of the goods. Or the payment will be made on receipt of the goods.