This forum is for income tax questions. You should look elsewhere for help with sales tax.
Since you are buying for resale, you should be able to give the provider some sort of resale certificate, which would allow them to not charge sales tax. North Carolina probably has a specific form for that purpose. Then you have to collect sales tax from your customers, based on the sales tax laws and rates for each customer's location. You're probably supposed to be doing that already, even though you have paid Washington sales tax.