i tried fiddling around the WePay button with some css and i think there are potentially two sources of problems, 1) being the browser (internet explorer) since firefox and chrome don't seem to have a problem
2) the actual WePay frame work, this would have to be potentially modified to accommodate the quirk in ie.
So here is a potential solution for you, with internet explorer you can you use conditional comments to potentially change the overall layout of your page just for individuals using ie. Here is an example:
Basically what is happening, if the browser is internet explorer the background color will be in blue while in other browsers this will be in lavender.
If you'd like to go with this approach i can try to update your page to accommodate this change due to ie....
hope this helps
God bless =)