Dunnes gift vouchers from St. Vincent's

Not sure if I'm asking in the right place but I recently found myself in hard times and contacted St.Vincent's, who gave me few Dunnes stores 20 euro gift vouchers. As far as I understand u can't use them online and only for groceries. My question is for those who have used them. Is there any limit I need to reach before being able to use the voucher,like for example, reach 50 euro to use 20 euro voucher or can I shop for 20 euros and use the voucher for 20 euros? There's nothing about that in terms and conditions. Can I use one voucher only per transaction or can I use for example 2 at the same time on 40 euro purchase? How it works?