the sticky cart option is an old feature that will not be developed any further as it causes more trouble than it’s worth with many themes (especially responsive ones as the sticky cart would prevent people on mobile devices from doing anything)

it’s only still there for some old or non-responsive sites, themes, legacy reasons.

i would advise you to abandon this in favour of the mini cart (enable that one instead in the widget/shortcode, with some more options to set in wppizza->layout).

In fact, as of wppizza v3.x the sticky cart will be removed in favour of the minicart (see widget and layout options)