The charming imp requires a Summoning level of 21 and a Dungeoneering level of 21 to purchase. If you have the charming imp in your inventory, he will pick up all charms dropped by any monsters that you kill - as long as you have space in your inventory. You can also tell him to keep charms if you do not want them at all. Charms that are kept by the imp will not appear on the ground or in your inventory. To tell the imp what charms to keep, right-click his inventory icon and choose 'Configure'.
Should you choose to stop the imp from picking up certain charms, you will gain a small amount of Summoning xp per charm. However, converting your charms to summoning experience is not recommended as each charm is worth far more experience used in a pouch than what can be obtained from the charming imp.