Try to use entry transaction(Setup>Sales>Entry transactions>Invoices and/or Orders) - you can hide them there.

After - you can use access codes to control which entry transactions are available to specific users.

I have a similar requirement, and have spent far too long trying to get it to work as I need.  I have a sales order screen (entry transaction) that I have configured for our sales team that has restricted access to certain fields but I am unable to default the screen to the user/users. I have access codes set up as best I can but we have other screens used by our bespoke web order process that will break if I restrict these in the same way-- so they are still all available to sales reps (sorry if this is confusing..).

Back in v6 I was able to use menu profiles to specify which users get which screen by default, but this no longer works.  I have set landing pages for users that I can assign screens to and that works but I can't do it from Navigation.  it seems very strange to me that this is no longer possible without setting access codes, which you would have to configure for every user in order to restrict a small subset.

I have a similar requirement, and have spent far too long trying to get it to work as I need.  I have a sales order screen (entry transaction) that I have configured for our sales team that has restricted access to certain fields but I am unable to default the screen to the user/users. I have access codes set up as best I can but we have other screens used by our bespoke web order process that will break if I restrict these in the same way-- so they are still all available to sales reps (sorry if this is confusing..).

Back in v6 I was able to use menu profiles to specify which users get which screen by default, but this no longer works.  I have set landing pages for users that I can assign screens to and that works but I can't do it from Navigation.  it seems very strange to me that this is no longer possible without setting access codes, which you would have to configure for every user in order to restrict a small subset.

System: SAGE X3 p9 u9 SEED demo folder Sales orders 
For base used transaction code ALL for sales order- copied it as SPEC entry transaction.
In entry transaction unchecked - tab Display field Margin and in tab Line changed Cost price to hidded.

ALL entry transaction:

SPEC entry transaction:

Cost price and margin fields arent listed any more. 

Create new Acces code - SPEC (named Special rights) in Setup>Users>Access codes

Now it is needed to assign for users. For that I created test user. In Setup>User>User> used USR08 as example for rights. And in tab Acces codes added Access code SPEC with all rights.. 

Now is needed to modify entry transactions to prevent TEST user  to use other transactions.

For faster solution - I set for all other entry transactions access code ADMIN, but for SPEC entry transaction I set access code SPEC.

For Test user will be available only one sales order entry transaction - SPEC and without visible cost price and margin fields. Same way it is repeatable also for sales invoices. 

... and yes - as it is only one available entry transaction it will be automatically selected for my test user.