You can use the crop tool in the LR develop module and set it at the desired aspect ratio before export, or you can use whatever tools Costco may provide to adjust your image inside the desired aspect ratio.

If you are confined to an 11x14 frame you must either remove part of the long side of the image or add to the narrow side of the image to get there. I cannot find a way to add empty space to the narrow side in LR. You can easily add canvas (mat), or distort the image in Photoshop, but I don't believe it possible in LR.

If you don't want to distort, mat, or crop, and you're not confined to the 11x14 frame, maybe the 12x18 is a better option.

Ok,yes,you can change the aspect ratio but you will end up with short fat trees or a stretch limo. Instead, you need to change "canvas size" to conform to a standard print size.

I crop my photos as 8x10" (4x5" ) to conform to USA paper and standard mats. However, wanted to send 4x6" drugstore prints to my sisters. OOPS, the drugstore would auto crop to fill the 6" side and cropped the 4x1.2= 4.8" to 4" losing 0.8" inches of my photo!! NO thank you. With 200 to customize, individual change was too time consuming... batch processing was the solution.

To solve the problem (excellent and free) Faststone.org "resize" allowed my to in one click change the batch canvas size to 4x6". The 1/2" could be at each end or could be as 1" at one end. I could leave as white/color or add a logo or comment or what ever. Of course the program will do individual photos as well as batch.

In a similar way you can fit your your photos into a standard only format and the computerized process will not crop. I advise not to battle with a one program dose all like PS or LR, use a dedicated program that knows it's specialty stuff like Faststone Resize.

http://www.faststone.org/FSResizerDetail.htm

You don't change the aspect ratio when exporting, you change it in the develop module when cropping. You must crop to that aspect ratio. After that, you export as a jpg but keep it at 360 ppi in Image Sizing dialog and keep the Quality at 100 in File Settings. You should be good to go.

If you don't have room to crop a little off the long edge of you image, then from now on you really need to leave more room when framing your shot in camera. In fact, it's always a good idea to leave room. Of course, this may not be possible if using a prime lens unless you back up and sometimes this can't be don't when there is action to be had.