I'm currently on an internship, where I have to send out invoices at the end of every week. After 1 month and a week of working, I finally receive my first cheque payment, however only received my first week's worth of salary. Does that mean I would only get paid a week's worth of payment after 30 days of each weekly invoice? I thought it's going to be the accumulated amount by the end of the month.
It probably means that you will get paid within 30 days of when you invoiced them. So if you sent out an invoice on January 1, you should get paid for whatever was on that invoice sometime around January 31.
This is not normal.
NET 30 terminology is for invoicing. This is simply a grace period to factor in confirming shipment quality, shipping, and various logistical issues involved in B2B transactions of large amount of goods.
However, this may be illegal as an employer paying their staff, contractor or not.
Contact your HR and see what's going on with your paycheck. If this is a one time occurrence, you can just let it slide. If they somehow justify delaying another 30 days for your wages (you are already working before you get paid, and now they're delaying it another 30), start looking at legal resources.