Once you have bought a lead, you can continue to contact the customer and send a quote without any further payments.
For example, if you buy a lead in order to send a message to a customer, and the customer then replies, you will not have to pay more credits to send your response.
The exception is if you buy a lead and then have your Plentific Credits refunded. When this happens the lead will become inaccessible, so you will not be able to continue using it. If you want to try and contact the customer again, you will need to buy the lead a second time.