Regarding shipping options

I am working on an application in which I want to use Zen-cart shopping cart. But I am facing problems in setting different shipping options. Actually I have to set different shipping options in different conditions. Example: if a customer purchases goods worth 1000 or above, then there will be no shipping charges applied but only if total weight is less than 5kgs. Above 5kgs shipping charges are applied.
Now if I want to put these conditions in the shopping cart, which file do I need to edit? Or if I want to set my own conditions, in which file can I write them?

IF you wanted to add this Free Shipping Option when Total >= 1000 and Weight <= 5 ... and have it show at the same time as the other shipping modules ... you could use the Free Shipping Options freeoptions and set the:
Total >= 1000
Weight <= 5

Now this will show in addition to your other shipping modules ...

Advantage: no coding changes

IF you want that to be the ONLY shipping option showing, then you would need to do custom coding changes to the other shipping modules to disable them ...

Can I use 2-3 shipping options at the same time in accordance with my requirements and if to modify existing options which files do I edit?

You need to be more specific about what you are trying to do ...

1. If total amount for a transaction exceeds Rs.1000, then there will be no shipping charges. (For total weight upto 5Kgs. Above 5Kgs shipping charges will be applicable.)
2. If total amount for consecutive 4 deliveries exceeds Rs.1500, then in that case for 4th delivery there will be no shipping charges. (Only if total weight for 4th transaction is upto 5 kg. If total weight is above 5Kgs, shipping charges are applicable.)

Now I want to implement the above shipping options. How do I go about it?
Can I use two or more shipping options at a time?

What do you mean when you say the 4th consecutive delivery?

Do you mean based on the 4th order? Or are you referring to something else?

On the Free Shipping Options freeoptions shipping module ... if you are wanting Total >= 1000 AND Weight <= 5 ... you will need to customize the code for:
/includes/modules/shipping/freeoptions.php

This IF statement right now is using the conditions based on OR

// final check for display of Free Options
      if ($this->ck_freeoptions_total or $this->ck_freeoptions_weight or $this->ck_freeoptions_items) {

4th delivery means 4th consecutive order..
If total amount for any 4 consecutive deliveries is more than 1000, then there will be no shipping charges applied for 4th order.. But only upto 5 kgs..

4th delivery means 4th consecutive order..
If total amount for any 4 consecutive deliveries is more than 1000, then there will be no shipping charges applied for 4th order.. But 4th order should be upto 5 kgs..

You would need to write a custom shipping module to check the last 4 orders of a customer to determine that:

1 there have been 4 orders

2 that 1 is over 1000 (are you saying all 4 or 1 of the last 4 is over 1000)

3 that the weight is less than 5

Something also to consider is are you only giving Free Shipping when this is true or is Free Shipping an Option when this is true ...