Aside from PayPal if you plan to sell items I suggest you sign up with the United States Postal Service and get an online account. You can do all your labels, print them off and pay for them online. The account keeps an activity history. All the tracking information is online and can be easily accessed as needed.
When make out the label you can have both you and the customer sent tracking notifications. When the USPS ships they send an email to your customer and you, both of you can tack the package. And you can keep addresses online too.
You can ship priority flat rate and the post office supplies the boxes and envelopes free.
If you use you own packaging you do need to enter the weight and sometimes box size.
I ship several packages a day, in the rare case when USPS drops the ball - you are at their mercy. That package may never be found and without tracking numbers USPS can do nothing. USPS can not tell you if any mail is on route. UPS and FedEX often can.
Good Luck, I am sorry your in this position. If you are becoming a regular sell, as you've learned record keeping is a must. Not just for billing and tracking but legally overall you need to keep records. The USPS and PayPal online records are very good tools, but still only one source of record keeping.