After paying at lancet got another outstanding

hi guys, such a poor thing to know about that after paying for out tests received a letter from NHLS saying that i owe them another couple of thousands, though we paid full payment which is quite a lot, then after 5 months or so we got a letter saying that we owe them the money.. but why did you guys not paid for them from what i paid from. for one test you guys charging such huge amount of money, which we did in india same one with better results explained for the price of 10 times lower... thought i didnt complained now i am complaining why would NHLS sending a statement, this is harsh on customers. you should sort it out with NHLS not from me.