I combine shipping - either request an invoice on the eBay desktop site (it does not work in the app) or send me a message. Sometimes it does not let you request an invoice online or it will say I do not offer combined shipping. 

Generally I can ship 4-5 comics via First Class, which will run between $4.19 - $5.49. More than 5 would have to go Priority Mail, which would go in a Flat Rate Envelope for around $7.75.