When importing if the error message Cast to String to Type Double is not Valid essentially means that there is an association that is missing. Try checking to see that there is a valid corresponding value for every product offer in the file. If there is a product01 there must be pricing, even if a 0 in the pricing01 field etc etc.
If you have orders that have more than 5 product offers that continue on the next line the shipping and tax fields must have a value even if the values are begin accounted for on the first line of the order. If this is the case then there should be a 0 in both fields in the additional lines.