If there is a discount coupon always activate shipping costs

[hupple]

I created a discount coupon with a value of 50 euros. If my customer  buys for  50 euros he will receive free shipping(if you buy for more than 30 euros) and will not have to pay anything because of the discount coupon of 50 euros. If he checks out then there is an error  notification "invalid amount". I am looking for some answers. Any of these posibilities would be fine:

1) The first and probably the best solution would be that not matter how big the discount coupon is there will always be the shipping costs (in case of an active discount). So the shipping costs should be activated when there is a discount and the shipping costs should not care if the customer buys for more than 30 euros or not. On the other hand if he buys for 80 euros there should be no shipping costs (as he buys for at least 30 euros).

2) another solution could be (but not the most favorable) that if the customer buys for 50 euros and use discount of 50 euros with free shipping costs he would not have to pay anything. This is the same scenario as written above. There should be however no "invalid amount" error and the error page should redirect the customer to  the thank you page

3) are there any other possibilities?

I have got one more question:
Is it possible that if the customer uses like 30 euros of his 50 euros discount that he can use the other 20 euros the next time he buys in the shop? I never figured that out...

Who can help me out?