Parcel size

Hi, I’m a new seller on eBay and have just packaged up my first order. The only issue is, it’s wider than 2cm so doesn’t class as a large letter as I’d listed it as. The parcel is around 14cm in length, 13cm in width and 6cm in depth. I am meant to be sending it tracked 48, but the small parcel option will cost me more than I sold the item for (including the postage fee I added). What do I do?! Any help or advice is greatly appreciated, thank you!