jQuery promises in sequence

Hi all.
This question follows this one: https://www.experts-exchange.com/questions/28923201/javascript-function-return-problem.html
I use jQuery promises to get some data and dinamically build an object to pass handlebars.compile. Here's the code:

function getFolders2() {
	var foldersContext = {};
	getFolderCardsNumber('PC').success(function (result) {
		var pcContext = {pc_cards_number: result};
		jQuery.extend(foldersContext, pcContext);
	});
	getFolderCardsNumber('SP').success(function (result) {
		var spContext = {sp_cards_number: result};
		jQuery.extend(foldersContext, spContext);
	});
	console.log('first context:');
	console.log(foldersContext);
	getCustomFolders.success(function (data) {
		var response = JSON.parse(data);
		if (response.success) {
			var folders = response.result;
			if (folders.length > 0) {
				var i = 0, l = folders.length;
				var id, description;
				for (i;i < l;i++) {
					id = folders[i].folder_id;
					description = folders[i].folder_description;
					getFolderCardsNumber(id).success(function (result) {
						var customFolders = {
							fd_cards_number: result,
							folder_id: id,
							folder_description: description
						};
						jQuery.extend(foldersContext, customFolders);
					});
				}
				console.log('second context:');
				console.log(foldersContext);
				var html = parseAndReturnHandlebarsRev2("folders2", foldersContext);
				console.log(html);
				jQuery('.all-folders').html(html);
			}
		}
	});
};

function getCustomFolders() {
	return jQuery.ajax({
		type: "GET",
		url: "includes/manage-folders/get_folders.json.php",
		data: {}
	});
};

function getFolderCardsNumber(folder) {
	return jQuery.ajax({
		type: 'post',
		url: 'includes/card-contacts/get_folder_cards_number.php',
		data: {folder: folder}
	});
};

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Well, the first two calls to external function getFolderCardsNumber() work fine and the object foldersContext is correctly built, but then I receive this error:

TypeError: getCustomFolders.success is not a function
    getCustomFolders.success(function (data) {

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Do I need a couple of fresh eyes?
Thank you.