The minimum amount you can add is currently $25, and there is a maximum per deposit of $500. If you need to add funds over $500, you can make another deposit, but there is a maximum balance limit per customer account of $3500.
Please note that you do not need to add funds to your account for most payments. The mentioned restrictions are only for adding funds to your account. You can pay invoices (any amount) directly with PayPal or a credit card.