You can set PayPal's standard shipping fields in the button's config under the Button-Type-Specific tab.
Shipping Amount - If 'Shipping Amount for Additional Items' is left empty, then the amount entered here will be charged only once despite the quantity of items selected. However, if an amount is also added for 'Shipping Amount for Additional Items,' then the amount listed here will be charged for the first item.
Shipping Amount for Additional Items - This will be charged per additional quantity of this item.
So for example, if $5 is entered into the "Shipping Amount" field and $1 for the "Shipping Amount for Addtional Items" and the customer purchases 5 items, then the shipping charge will be $9 (($5x1) + ($1x4)=$9).