My intent is to send more letters/postcards as well as follow-up with all potential deals (something I was lazy about doing previously).

Your letter or postcard serves as an introduction.

Their prices are very cheap ($.40/ postcard & $.50/yellow letter when sending over 1000) but I have not seen any reviews on them. 

"I have like 8 different postcards on my table, though now that we're on the phone, I'll tell you more about the house..."

And I used Click2mail for the postcards.

Which postcard/bulk mailing company is the cheapest/most effective with a customized design?

If you go to yellowletters.com (picking a letter service at random, not picking on YL);  and buy their list of absentee or vacant you will find lots and lots of those post cards return undeliverable etc. 

I’m learning to wholesale and currently taking a wholesaling class, and I have sent out my first postcards. 

Find areas to farm and send postcards via the USPS's Every Door Direct Mail (EDDM). 

So as I am learning about shotgun marketing I am hearing that the process in general could be quite expensive yet no one has explained specifically what parts of the shotgun style of marketing actually IS expensive.Besides the cost of stamps, post cards and envelopes, (and I'm told I also need to purchase a $150 permit from the US postal service in order to send bulk mail) what other expenses do I need to take into consideration?