Give this a try:
Remember: remove extra classes such as
card-block justify-content-center align-items-center
in the section tag.
<div class="row justify-content-sm-center">
<div class="col-sm-5">
<section class="ContractTemplateDetails mt-sm-0 unselectable mr-sm-2 card h-100" data-template-id="18">
<!-- Card content -->
</section>
</div>
<div class="col-sm-5">
<section class="ContractTemplateDetails mt-sm-0 unselectable mr-sm-2 checked card h-100" data-template-id="18">
<!-- Card content -->
</section>
</div>
</div>
See the Demo HERE
Updated
<section class="ContractTemplateDetails mt-sm-0 unselectable mr-sm-2 card h-100" data-template-id="18">
<div class="card-text-content mb-auto">
<div class="contract-image"><img class="" src="http://freevector.co/wp-content/uploads/2010/05/29358-toy-car-outline.png" alt="Contract Basic"></div>
<h4 class="contract-title">Contract Basic</h4>
<ul class="contract-list">
<li>Lorem ipsum dolor sit amet consectetur, adipisicing elit. Impedit non </li>
<li>Lorem ipsum dolor sit amet consectetur, adipisicing elit. Impedit non.</li>
<li>Lorem ipsum dolor sit amet consectetur, adipisicing elit. Impedit non </li>
<li>Lorem ipsum dolor sit amet consectetur, adipisicing elit. Impedit non omfattet af serviceaftalen.</li>
</ul>
</div>
<div class="contract-price mt-auto">
<h2 class="component-margin-top-small">205,00 kr./md.</h2>
</div>
<div class="SamCheckbox custom-control custom-checkbox"><input type="checkbox" class="custom-control-input" id="sam-check-undefined" value="18"><i class="custom-control-indicator"></i></div>
</section>