given you statement the sentence will be:

If the supplier had signed the agreement he would deliver the goods.

Hope this makes it clear.

If the supplier has signed a binding contract, he will deliver the goods.

What's the meaning of the sentence? In case, if the supplier has signed a binding contract that he is under the obligation to deliver the goods. Here delivery of goods is the outcome of signing the contract. Now, "the supplier has signed the contract" makes it definite that at some unspecified times in the past, the supplier for sure has signed the contract. But "if" makes it conditional now. So we are not sure whether the supplier has actually signed the contract. There can be two outcome of the sentence: If the supplier has signed the contract, he will deliver the good. If has not, then he will not deliver the goods.

You ask why can't we write the sentence as: If the supplier has signed a binding contract, he would deliver the goods. Now, the assessment that since the contract has been signed in the past some time, the goods have also been delivered in the past some time. But we do not know for sure whether the contract has been signed or not because of the presence of "if". So how can we say that the goods have been delivered in the past? Here, the sentence is stating a condition. If the contract has been signed, then the supplier will have to deliver the goods. The one thing we know for sure is that the goods have not been delivered.

Take an example. If you have practiced well, you will excel. From this sentence, what we know for sure is that "you" has not excelled yet which depends on the practice. So if the practice has already been done, then the action of excelling is sure to follow in near future because this action has not taken place yet, irrespective of practice.

Thanks.