I see that delivery time for the metal I want is three weeks. Does it mean I can fix the price now and pay three weeks later?

Buying and selling of metals, is executed on a cash only basis. As soon as we order from suppliers, they expect us to receive the funds within one business day. Likewise, for our customers, they would also have to send us the payment as soon as possible, even if delivery will take three or even four weeks. Once a customer submits an order, you can assume that we have covered your order, and that we are awaiting payment from them.