SVP's national retail development manager Dermot McGilloway said the society is "very dependent" on the contribution from the shops.
He also said he believes it was "discriminatory" that they were not deemed essential, as they are "not only a fundraising service but they also provide essential goods".
"Imagine telling people that clothes aren't essential?" he said. "Some people don't have access to a credit card or the internet - digital poverty is a thing.
"It was the big multi-national retailers that were best to do online and the small community charity shop at the bottom of the street that provided people with affordable clothing and gave rough sleepers emergency assistance, but they were gone."
Mr McGilloway said that due to the Covid pandemic, the gap is widening between "those that have and those that have not". "The retail shops are the gateway to the eco-system for SVP, so to have them closed for 20 weeks and saying it's not an essential service is just perverse," he added.
Julieann Beale, who manages the SVP shop in Tubbercurry, Co Sligo, said the stores are not only about fundraising, they create a community of volunteers, customers and donors.
Many different people volunteer in the shop, including asylum-seekers and people doing Tús or Solas courses, and Ms Beale said it is "incredible to watch their confidence grow as they help out in the shop".
"Without places like SVP at the time, asylum-seekers weren't allowed to work and they weren't allowed to go to college so there was nothing for them to do," she said.
"They were getting €19 a week, with no way of meeting anybody or integrating into the local society, so we trained them in different areas to help them in integrating further.
"Two of the girls are now qualified nurses - they turned their lives around.
"I have had the pleasure of watching people arrive in the country and go through that system. I was with them when they were crying, but I've seen them turn their lives around.
"I've seen them go to college and get an education, and one guy is actually down in LIT doing his masters. It's not about me, but in these kinds of shops we do care about the people that work in them."
Ms Beale said the customers in Tubbercurry also rely heavily on the shop for a sense of community, which they missed during the various lockdowns.
"Our shop opening again this year saved an awful lot of people in our local town because suicide can be pretty high in these rural areas," she said.