Use the ARPostInvoices process.

Oops, I mean ARPostReceiptsAndAdjustments

It doesn't work for my case, after creating an order, I would use the receipt number to create a receipt using ARPostReciptsAndAdjustments which works but the receipt doesn't get posted but left in an open state i.e Status: "Open".

How can I post it directly and Status: "Posted"?

Thanks.

The ARReceiptAndAdjustmentBatches (HTTP POST) endpoint creates batches. You cannot post (Sage post) them using this endpoint in an HTTP POST. It might be possible at this point to add the batch status of ReadyToPost, but I have not tried. To post them you must do a subsequent call to ARReceiptAndAdjustmentBatches, this time with an HTTP PATCH and you pass in the batch number and set the batch ReadyToPost. For example:

http://localhost/Sage300WebApi/v1.0/-/SAMLTD/AR/ARReceiptAndAdjustmentBatches(58)
{
    "BatchNumber": 58,
    "BatchStatus": "ReadyToPost"
}

Then you must do a further HTTP Post to localhost/.../ARPostReceiptsAndAdjustments('$process')

This will process any batches that are ready to post.

Thanks a lot, this works perfectly.

I would also like to ask why pricing doesn't show on get all item pricing but shows when getting a individual item pricing.

It doesn't show here (get all item pricing)

But shows here (get a single item pricing).

Is there any way I can resolve this?

Thanks.

You'd need to contact 300 developer support for answers to that. The web API is what it is, limitations and all.