Bestmed btry to collect money 1 year after I left them

I've been with Bestmed for 2 years and then joined Discovery with my husband. When I informed Bestmed that I won't be using their service anymore nothing was said about outstanding money then last week I got an email after 1 year with an invoice of R1816.45 telling me I have 14 days to pay or I will be handed over to lawyers. Bestmed is the worst medical aid ever they always have problems and strories when they have to pay out but then they send me an Invoice a year later and not when I tell them I'm leaving them to go on a medic aid with my husband. I insist Bestmed to pay back my money it's not my fault they are not doing their work and then want to collect money from clients a year later just before Christmas time