I didn’t really understand your question. Buyers do not send custom offers. It is the sellers who send them.
The offer that you send while responding to a buyer request (BR) is also a type of custom offer. If the buyer is interested in your BR offer, it is possible for them to directly accept it without contacting you via your inbox. When that happens, you get notified of an order and you can then start working on it by going to the order page. If, on the other hand, a buyer contacts you and either sends you a custom order or says that they are interested in working with you, you can send them a custom offer. I have already explained how to create a custom offer in post #2.
By the way, if you are looking for help on the forum, please create your own forum post (provided there are no pre-existing threads which already have the info. you’re looking for) instead of continuing in an old thread.