Newbie accounting student here, need help with some questions.

Hi guys, newbie accounting student here. So i'm practicing recording transactions and I'm stumped with the following transactions:

Sold merchandise on cash to Heya Merchandising, 20,000. Freight paid, 1,000. My entry here is:

Cash 20,0000

Sales 20,000

Transportation Out 1,000

 Cash                     1,000

Does freight paid mean I, the seller, pay for the transportation? Did I do it correctly?

I also wanna know if this is an error from the book I am studying:

So Feb 1. you purchased merch. from small distributor, 200,000. 15/5, 5/10/ n/30. F.O.B Shipping point. Freight prepaid 2,000.

Feb. 6, you received credit memo from smaller distribution, 1,500. Isn't the entry,

Sales Returns and Allowances 1,500

 Accounts Receivable                   1,500

but you are the buyer and not the seller? Why would I have sales returns and allowances? Am I wrong?

My last question is this transaction:

Sold merchandise to the following customers:

• Cash basis, 60,000

• On account with 10%, 30-day notes, 50,000. Terms: 5, 2/10, n/30. F.O.B destination, freight collect, 1,400.

• One open account, 60,000. Terms 10/15, 5/20, n/30. F.O.B destination, freight prepaid, 2,000.

I know how to answer the freight and the terms but I do not understand the second and third one. "On account with 10%, 30-day notes" and "On open account" , can anyone explain this to me? It is my first time to encounter such transactions. I tried searching on the internet and reading my book but I haven't seen any tips on how to deal with such transactions. Also, sorry guys for my English. Thank you guys in advance.