Hi, since you have a few questions I will number my reply.
1. You can insert the return shipping cost in any of the red expense section. For example, you could add it to the postage column, or under packaging supplies... all of this section is subtracted from your profit.
2. If you downloaded the sale before the item had been returned, and a FVF was populated, it will not overwrite that value. If you want to get the updated value, first reset the download duration in the Account Info tab to go back far enough for that sale. When you re download your sales after that it will refresh any BLANK cells for that sale. So if you want to update FVF, you could delete the current value there (click that cell and hit the delete key to clear contents)... or you could delete the whole sale (click the FULL SCREEN button to display the row labels, right click, and delete the entire row) and when you re download it will fill in a new row.
3. "#####" is excels way of showing that the whole number will not fit in the display area of that cell. Click the FULL SCREEN button to display the column lettering, and you can drag the column to be wider.
I hope that helps!