Well, technically, you should use future simple: I'm sure you WILL RECEIVE something in the post in the next couple of days. Because the implication is that you'll receive something ONCE, and the continuous tense indicates a more repeated event, or something that lasts for some time (for example "you will be eating", "you will be thinking about me") Having said that, future continuous is sometimes used in that context too, so "I'm sure you WILL BE RECEIVING something in the post in the next couple of days." wouldn't be totally weird to hear. I think there is a slight difference, in that "you will receive" sounds a LOT more certain than "you will be receiving", so if you wanted to convey a firm promise, you'd say "you will receive", and if you were less sure, or didn't want to fully commit, then you might say "you will be receiving".

Diego is correct about future versus future continuous. As far as present continuous, although we often use the present continuous to talk about future plans, we cannot use it to talk about future predictions.

You need to say "you'll receive" because you're talking about something that WILL happen so you have to say in future tense.