Hi MarleneM,
I'm actually not so sure the calculation issue you mentioned is related to your inability to enter package dimensions when listing these items, especially given that it is not even possible to enter package dimensions for listings which only offer Parcel Select shipping when creating them directly through the Sell your Item form on the eBay website.
Also, our system does not actually calculate postage for your items as you suggested. We simply submit the package and shipping service details you enter along with your listings and eBay's shipping calculator calculates the postage based on those details.
However, if you feel the lack of dimensions is the cause of the discrepancy, it appears that you can add dimensions to listings posted using this service through the eBay Revise your Item form while they are active as I mentioned in my previous reply.
If you have not done so already, it would also be a good idea to contact eBay's Customer Support team and see if they can provide you with any additional information on why the shipping calculations for these items did not match what it actually cost to purchase your labels.
Although it is definitely strange that eBay allows dimensions to be added when using this service through their Revise your Item, I do not currently believe any changes are necessary on our side since the behavior on our site currently matches the eBay Sell your Item form in this regard.
I hope this adds some clarity to the situation.
-Mike