I have been trying to buy spectacles on line but am having problems entering the prescription can anyone help?

I need bi-focal lenses and the problem is with the near vision entry. The values in the Sph boxes on my prescription are +4.50 and +7.50 for right and left respectively but the drop down menus on the web sites only go to +3.50. Also I see from previous prescription that the values are much lower at +1.75 for both R and L. I do not think my vision has changed greatly. Can anyone help a confused old man?