The Side by Side Ukes band entertained shoppers outside the Morrisons store in Wellington.
Zac has an extremely rare form of childhood leukaemia called Near Haploid, and needs to travel for specialist treatment unavailable on the NHS.
People across the world have come together to help to raise the £500,000 needed to get him to America, where he will be able to receive the specialist CAR-T therapy in Philadelphia.
Zac’s family announced last week that the total had been reached after a mysterious £100,000 donation to the youngster’s cause.
Celebrities backing the campaign have included Simon Cowell, Jonathan Ross, Jermain Defoe and Mark Knopfler.
Zac's parents, Hannah and Mark, are now making preparations to take him to Philadelphia where he can undergo CAR-T therapy unavailable on the NHS for his rare form of leukaemia, called near haploid.
They are continuing with fundraising to cover the costs of support treatment after Zac's therapy.
How you can continue to help:
Anyone who wants to support the fundraising campaign should visit justgiving.com/crowdfunding/zac-oliver
People can also join Zac's Facebook support page which is called 'Zachariah's's fight against Acute Lymphoblastic Leukaemia'.
People can also donate using their mobile phone by texting ZACH75 £1 to 70070.
The Side by Side Ukes are a group of retired people who perform at functions across the region to raise money for charity. They jokingly refer to themselves as recycled teenagers.
When they heard about Zac, they knew they wanted to get involved in the effort to raise money for him.
Jack Pritchard of the Side by Side Ukes, said it usually takes six months to organise a street gig, but that managers at the Morrisons store had worked hard to make an exception for Zac.
“It was very good,” Jack said. “We raised £178 for Zac.
“There were four of us out of our total eight because it was a bit short notice, and we only did two hours because that is the maximum our voices can do.
“We’re very grateful to the store managers Henry Brindle and Jason Bagley who gave us permission at the last minute, because usually it takes about six months to sort that kind of thing.”