You could use one of two ways to make sure your shopper receives a discount on purchase from the BlueSnap catalog price:
1) implement a coupon. store it in your system associated with the shopper you created. Then, use the orders web service to place an order for the shopper with the discount:
https://ws.bluesnap.com/services/2/orders POST
<order xmlns="http://ws.plimus.com">
<ordering-shopper>
<shopper-id>19575992</shopper-id> -- the shopper ID you prepared in advance
<web-info>
<ip>62.219.121.253</ip>
<remote-host>www.merchant.com</remote-host>
<user-agent>Mozilla/5.0 (Linux; X11)</user-agent>
</web-info>
</ordering-shopper>
<cart>
<cart-item>
<sku>
<sku-id>2152762</sku-id> -- a product that has catalog price of 10 usd
</sku>
<quantity>1</quantity>
</cart-item>
<coupons>
<coupon>30-percent-off-code</coupon> -- the coupon code you made for the shopper.
</coupons>
</cart>
<expected-total-price>
<amount>7.00</amount> -- the price for this shopper after the discount
<currency>USD</currency>
</expected-total-price>
</order>
2) Use override price. In this case, you can control exactly how much of a discount the shopper gets, regardless of the catalog price:
<order xmlns="http://ws.plimus.com">
<ordering-shopper>
<shopper-id>19575992</shopper-id>
<web-info>
<ip>62.219.121.253</ip>
<remote-host>www.merchant.com</remote-host>
<user-agent>Mozilla/5.0 (Linux; X11)</user-agent>
</web-info>
</ordering-shopper>
<cart>
<cart-item>
<sku>
<sku-id>2152762</sku-id>
<sku-charge-price>
<charge-type>initial</charge-type>
<amount>7.00</amount>
<currency>USD</currency>
</sku-charge-price>
</sku>
<quantity>1</quantity>
</cart-item>
</cart>
<expected-total-price>
<amount>7.00</amount>
<currency>USD</currency>
</expected-total-price>
</order>
In both cases, the product that costs 10 USD for anyone else will be sold to the shopper you selected for 7 USD.