Override the popup close method with one that doesn't fade out.

custom.js

jQuery(document).ready(function () {
    setTimeout(function () {
        jQuery.fn.popmake.close = function (callback) {
            return this.each(function () {
                var $popup     = PUM.getPopup(this),
                    $container = $popup.popmake('getContainer'),
                    $close     = $popup.popmake('getClose').add($('.popmake-close', $popup).not($popup.popmake('getClose')));

                $popup.trigger('pumBeforeClose');

                if ($popup.hasClass('preventClose') || $container.hasClass('preventClose')) {
                    $popup
                        .removeClass('preventClose')
                        .trigger('pumClosePrevented');

                    return this;
                }

                $container
                    .fadeOut(0, function () {

                        if ($popup.is(":visible")) {
                            $popup.fadeOut(0);
                        }

                        $(window).off('keyup.popmake');

                        $popup.off('click.popmake');

                        $close.off('click.popmake');

                        // Only re-enable scrolling for the document when the last popup has closed.
                        if ($('.pum-active').length === 1) {
                            $('html')
                                .removeClass('pum-open')
                                .removeClass('pum-open-scrollable')
                                .removeClass('pum-open-overlay')
                                .removeClass('pum-open-overlay-disabled')
                                .removeClass('pum-open-fixed');
                        }

                        $popup
                            .removeClass('pum-active')
                            .trigger('pumAfterClose');

                        // TODO: Move this to its own event binding to keep this method clean and simple.
                        $container.find('iframe').filter('[src*="youtube"],[src*="vimeo"]').each(function () {
                            var $iframe = $(this),
                                src     = $iframe.attr('src'),
                                // Remove autoplay so video doesn't start playing again.
                                new_src = src.replace('autoplay=1', '1=1');

                            if (new_src !== src) {
                                src = new_src;
                            }

                            $iframe.prop('src', src);
                        });

                        // TODO: Move this to its own event binding to keep this method clean and simple.
                        $container.find('video').each(function () {
                            this.pause();
                        });

                        // Fire user passed callback.
                        if (callback !== undefined) {
                            callback();
                            // TODO Test this new method. Then remove the above.
                            //callback.apply(this);
                        }
                    });
                return this;
            });
        };
    }, 1000);
});