This will depend on the supplier and product. Normally, you will be expected to telephone the supplier to provide payment details. By doing that your order will be confirmed. We will notify you on the website of supplier contact details as soon as the supplier is chosen.
If however you are already a customer of the supplier that is selected, there is a good chance that they deliver without prior payment, in which case you will receive an invoice and be expected to pay promptly.
Some suppliers are happy to deliver first and invoice later.