Fixed order amount/price for other cultures

For some values, .ToString() on Double may convert it to scientific format (X.Xe-10 etc.). This is fixed by "F8", as for CEX.io there are 8 decimal places.
What's more important, in some countries, .ToString() on Double will replace dot with comma (0,123 etc.), therefore using InvariantCulture is necessary.
Merge pull request #1 from WRonX/patch-1

Fixed order amount/price for other cultures