var startSize = $('.product-spoiler').css('border-bottom-left-radius') || 0,
endSize = 30,
size,
duration,
visible;
$('.product-spoiler').on('click', function () {
$(this).find('.show-block').slideToggle(duration);
visible = $(this).find('.show-block').is(':visible');
size = visible ?startSize :endSize;
$(this).animate({
borderBottomLeftRadius: size,
borderBottomRightRadius: size
}, duration);
});
$(...).slideToggle({
complete: function() { твой код }
});