Buy Now buttons are best used for services where as Shopping Cart buttons are best used for products.
Item name - Name/description of the product being sold.
Item ID (optional) - SKU for the product
Item Price - Amount to charge for this item (not applicable if using a price per option dropdown). You can allow the user to set a custom amount by checking "Allow user to set amount." Minimum and maximum amounts can be set to restrict the user's input.
Item quantity - The number of items per button. This is disabled for Buy Now buttons by default unless you check "Add a quantity option to this service." You can allow the user to set a custom quantity by checking "Allow user to set quantity" and define a minimum and maximum to restrict their input. For Buy Now buttons, if you want the quantity field to only be editable on your site and not on PayPal's site (for example if you are using discounts), then check "Restrict quantity adjustment to this site." If you want to restrict a user from adjusting the quantity for Shopping Cart buttons in the built-in cart, then uncheck "Allow cart quantity adjustment."
Handling fee ($) (optional) - an amount to charge as a handling fee one time regardless of quantity
Handling fee rate (%) (optional) - a percentage of the entire amount (including amount * quantity) to charge as a handling fee.
Shipping amount (optional) - an amount to charge as shipping for this item. If Shipping amount for additional items is left empty, then this will be charged as a flat rate regardless of quantity.
Shipping amount for additional items (optional) - If set, this will be charged per each additional quantity of this item. The first item will be charged the amount set for Shipping amount.
Advanced shipping fee (optional) - Assigns an advanced shipping fee profile to this button.
Tax amount ($) (optional) - A fixed amount to charge as tax regardless of the item's price. This will override any tax rate set.
Tax rate (%) (optional) - Percentage of the item's total amount to charge as tax.
Apply tax to S&H (optional) - If set to yes, shipping and handling will be included in the amount when calculating tax rate.
Item discount amount (optional) - A flat amount given as a discount for this item regardless of quantity. This will override discount rate fields.
Discount amount for additional items (optional) - If set, each additional quantity of this item will be discounted by the set amount. The first item will be discounted by the amount set for Item discount amount.
Item discount rate (optional) - Same as Item discount amount except the discount will be calculated as a percentage of the entire item's amount.
Discount rate for additional items (optional) - Same as Discount amount for additional items except the discount will be calculated as a percantage of the entire item's amount.
Advanced Discount (optional) - Assigns an advanced discount to this button.
Item weight (optional) - Sets the weight for this item. You can choose between pounds and kilograms.