hello, each theme which is marked "_german" uses a full ajax refresh for all of the inputs - whole theme runs in a single form.
pls see the themes at the display tab and choose a one that would suit you the best.
some theme are compatible with "Ajaxify Cart" at the Display tab (german themes have this automatically enabled):
try this option to see if it enables the Ajax refresh of the product lines.
if the option above is enabled, different files are used to render quantity update controls and the coupon controls (so they don't include another form element).
if you needed the Icetheme (let us know which one exactly) to be 100 percent compatible with this feature, let us know and we can run tests for this. some themes which we did not find fully compatible with this option have this option disabled by it's own config override in /components/com_onepage/themes/YOUR OPC THEME/overrides/onepage.cfg.php and it may need to get deleted or adjusted to allow such feature:
(remove or alter this line to create theme's default settings)
best regards, stan