You can try the options given below. The chart will help you make a conclusion which email marketing software company you want to choose.
All though they are numerous companies but you can try any of these above companies. I found this chart very interesting and BinaryClues and Easy Sendy Pro are the best ones in my opinion.
In your pay as you go model also you can decide how many min. email you want to send to your customers? As this will not help you save cost even buying a services for sending min. no. of emails like from Mailchimp would be costlier to you in comparison to buying a bulk package from BinaryClues.(See report)
Thus, I would recommend you to go for a Bulk email package as any how these are cheaper and will cost you in...