переведите на англ. пожалуйста, несколько предложений

I am sorry, I can't accept this offer. You can place a bid on this site, and I'll send this item to Canada. If the shipment will be cheap, I'll pay the difference to you.
Наверное, как-то так.

I'm sorry, but I can't accept this proposal. You can place your stake on this site and I will be pleased to send you this item to Canada. If I manage to send the parcel cheaper I'll send the difference in the delivery charge back to you.
(If the delivery charge happens to be lower than I expect, I'll return you the difference in money.)

I am sorry but I can't accept such offer. You can place your stake on this site and I will send this item to you, in Canada with pleasure. And if sending of the parcel is cheaper, I will give back a part of money to you.
вообще-то для перевода деловой переписки нужно обращаться к профессиональному переводчику.

I regret, but I can not accept such offer. You can make the stake at this site and I shall gladly send you this item to Canada and if I shall succeed to send the parcel with cheaper terms I shall return the part of money for delivery.