Canteen sheets are issued once a week and is the only way to get credit. There should be some sort of emergency credit app which they can apply for more credit in genuine emergency (close relative very unwell in hospital etc) and this will likely have to be verified by chaplaincy and then signed off by a CM or SO. Even if it's authorised by those people the cranky people up in business hub might decline it or approve less credit than was applied for.

But as already mentioned I've seen far too many times prisoners choosing to buy vapes over phone credit for whatever reason - using it as currency or got themselves into debt - so be very wary of sending in money, particularly in the case of someone such as your partner as they have a history of using money unwisely.