Anyone who cannot leave home may be able to ask a trusted friend or volunteer to withdraw cash at any Post Office using a single-use voucher.
The Post Office scheme is being extended and offered to all banks, building societies and credit unions.
If the bank allows it, someone can ask for a one-time barcode sent via text, email or post for a stipulated amount.
A trusted friend or volunteer can exchange the voucher for the cash requested.
Previously, only a named individual, such as a carer, could collect cash in this way on someone's behalf. Now any trusted neighbour or volunteer can do so.
The idea of the Payout Now scheme is to allow people who are shielded or self-isolating, mainly elderly, to maintain access to cash without having to hand over a debit card and Pin to somebody else.
They tell their bank exactly how much they want to withdraw from their account, up to a limit set by the bank, and allow a family member, trusted friend or volunteer to collect it on their behalf in exchange for the voucher.
Martin Kearsley, banking director at the Post Office, said: "Being able to easily access cash is a vital service for older people and those self-isolating.
"[This] means they can access cash quickly and securely to repay someone for a helpful service like shopping, or simply manage their finances, providing peace of mind that cash can be securely sourced with the help of any trusted helper."
A service that allows vulnerable customers to contact their bank and arrange to cash a cheque at a Post Office branch is also being sped up.
Under the Fast PACE system, the customer should contact their bank and check they can use the service. They would then write a cheque to "The Post Office", print the name on the back of the cheque of the person collecting it for them and sign that side too.
That individual can then collect the cash from a Post Office branch after their ID is verified. At its fastest, the whole process from the initial call to the cash being collected could take a day.
"Anyone collecting cash on behalf of another person must remember to practise safe distancing and should consider arranging with the recipient how the cash can be safely handed over - perhaps through a person's letterbox, for example," Mr Kearsley added.
Both schemes come with a warning that people should only use friends and volunteers who are completely trusted, they should only withdraw cash they really need, and they should not be put under any pressure to do so.
There have been reports of fraudsters offering to shop for people who cannot leave the home, but who steal any money they are given, or take money from accounts after a Pin is handed over.