Discount on Quantities that integrate with other apps

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Hi there, in my store I have a quite big problem.
I decided for each product to create a discount on quantities so for example
Product A = 30,00 Dollars
2 X Product A = 55,00 Dollars
3 x Product A = 70,00 Dollars

The problem is that these are 3 separated products. 
Now if someone goes on cart and manually add 3 packets of product A, and not the product itself, ends up paying 90,00 dollars instead of 70,00. 

Many customers are complaining about it.

I tried to add some apps about automatic discount quantities, the problems are essentially two:
1) Every app I tried **bleep**ed up some part of my shopify, either the cart, the checkout or the images of the products. They are all extremely buggy and every single time I had to pay some guy to fix what was not working anymore.
2) When I found one that was working without any problem, it didn't integrate with other apps. 

For example I have a bar that says shipping over 49,00 is free. 
I have gifts for orders over 99 and 199. 
However the quantity apps couldn't communicate with the others, so the other apps where always calculating the total of the product as price and not the discounted one. 

So if somebody was ordering 2 products for a total discounted of 47, but the total not discounted was 55, the app was giving free shipping also if it shouldn't. 

How and what can I do ? 

I'm quite sure there are thousands of stores with automatic discount quantities working properly. Thank you