Row 197-198:
From
$products[$key]->_tmp_var_top_buttons .= '<span class="ca_qty_input">'.\JText::_('JSHOP_QUANTITY') .
': <input type="text" size="2" value="' . $value . '" name="quantity" class="input-mini" /></span>';
To
$products[$key]->_tmp_var_top_buttons .= '<span class="ca_qty_input">'.\JText::_('JSHOP_QUANTITY') .
': <input type="number" min="0" size="2" value="' . $value . '" name="quantity" class="input-mini" /></span>';
With this change you get a +/- button in the field.