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.