Best Shopping Cart?

I’m adding on to a website I designed a few years back to include a payment process for an insurance company.

They want customers to be able to log on with a bill they have recieved in the post and to pay from their website.

My experience of carts have been woocommerce, just wondering the best system to put in place.

So in Short

Customer gets bill
Customer goes to site, inserts reference number and amount owed
Customer clicks Pay

The site is on windows hosting with dedicated IP and SSL cert in place and a wordpress theme.

Thanks in advance