Hi larries_attic,
The reason that you are receiving an error message when you try to enter fractions or decimals in the "Package Dimensions" fields is because those fields only accept whole numbers. Therefore, to ensure that the shipping calculation does not come out too low, I recommend rounding up any fractions to the nearest whole number.
If you have any additional questions or concerns regarding this matter, please file a support request using the appropriate link on our help page: http://www.auctiva.com/free-auction-software-help/help.aspx
-Mike