Can a client add credit to their account?
You can enable clients to top up by navigating to System > Settings > Accounting Settings > Client Balances and setting 'Client top-up permissions' to 'Can top up'. A client can then top up by going to their Dashboard and clicking the 'Balance Top Up' button. Enter the amount you would like to top up then pay with Stripe. You should notice that your balance has increased by the top up amount.
Is there a way for clients to choose pre-selected top up amounts?
Unfortunately users wouldn't have the option of choosing pre-set amounts to top-up their account by. When topping up, clients would need to enter an amount themselves when adding that credit to their account.
Can I disable the Client Top-Up Function?
You can disable clients to top up by navigating to System > Settings > Accounting Settings > General and change the 'Client top-up permissions' setting to 'Can not top up'.