请回答3个问题

不必翻译 回答问题

1.on what occasions in the course of contract fulfillment does the seller need to communicate with the buyer concerning payment ？
2.by what forms do the seller and the buyer communicate with each other about payment in fulfillment of the contract ？
3.which manner is the more often used one，wtiting or oral？

Is too professional, I can not answer incompetence