Greg-Mega

You sure that's right? Here in Australia (GST is 10%) 
Inclusive would be:
Subtotal $3.00
GST Portion (10%): $0.30
Total: $3.00
Exclusive would be:
Subtotal: $2.40
GST Portion (10%): $0.24
Total: $2.64
In your example you tell me 20% of  £3 is £0.60 but it also tells me 20% of  £2.40 is $0.60 and it's not, it's  £0.48.
EDIT: If the Blesta developers want someone from Australia that has a good understanding of Australian taxation I'll stick my hand up. Sales tax really shouldn't be that complicated though you've done the hard part in appeasing for cascading taxes it's just now a matter of straightening it out in my opinion.

Do you have SPF & DKIM records for your domain? Gmail has got quite picky about these in the last 6 months along with O365.
As you've removed your domain name from the headers I was unable to check this for you.

Could be way off the mark here but is your cron set up in cPanel? you're probably receiving the cron email notification from that? it's at the very top of the cron jobs screen.
 
http://docs.cpanel.net/twiki/bin/view/AllDocumentation/CpanelDocs/CronJobs#Adding a cron email
 
This sends you an email every time a cron job is executed and includes its output if any, just get rid of it (leave blank and select update) to set it as blank.

It'd be good if it was configurable so this functionality could be enabled, disabled or enabled but only on paid invoices ie. invoices with transactions.

Could be way off the mark here but is your cron set up in cPanel? you're probably receiving the cron email notification from that? it's at the very top of the cron jobs screen.
 
http://docs.cpanel.net/twiki/bin/view/AllDocumentation/CpanelDocs/CronJobs#Adding a cron email
 
This sends you an email every time a cron job is executed and includes its output if any, just get rid of it (leave blank and select update) to set it as blank.

I've ran into similar issues in the past, for the most part I always have a contact person but I know in the past I've used:
 
Given name: Claim
Surname: #CLAIMNUMBER
 
For an insurance job, I've also put the department in:
 
Given name: ICT
Surname: Department
 
Which tends to make things easier in government and large companies, so they know what pool of money it comes out of. And I think once I even did:
 
Given name: Att:
Surname: N/A
 
And: 
 
First name: PO Number
Surname: #PONUMBER
 
I think if this was a feature, I'd still want the given name, surname fields to be mandatory for a client registration and perhaps not mandatory for admins. It's always handy to have a contact person! If that's a bit all over the place I'd probably leave it as is and deal with it.

I was stuck at work last Sunday restoring a server from a backup and had a couple hours to kill so I pieced this together for PIN Payments (http://pin.net.au/).
 
For those that don't know it's almost identical to Stripe but has been available in Australia for a couple of years where Stripe hasn't been (in Beta now to a small group of users).
 
Currently supports Charge and Refund, I will look at token based charges eventually.
Supports all currencies PIN supports (AUD, USD, NZD, SGD, EUR and GBP)
Supports cards PIN supports (MC and Visa, picks up on others and throws appropriate error)
 
Not really anything to show in the way of screenshots really:
 

 

 
A little bit more refinement around failed transactions needs to be done so the messages are more informative but it does work.
 
Download: pin_payments V1.0.0.zip
 
 

I was stuck at work last Sunday restoring a server from a backup and had a couple hours to kill so I pieced this together for PIN Payments (http://pin.net.au/).
 
For those that don't know it's almost identical to Stripe but has been available in Australia for a couple of years where Stripe hasn't been (in Beta now to a small group of users).
 
Currently supports Charge and Refund, I will look at token based charges eventually.
Supports all currencies PIN supports (AUD, USD, NZD, SGD, EUR and GBP)
Supports cards PIN supports (MC and Visa, picks up on others and throws appropriate error)
 
Not really anything to show in the way of screenshots really:
 

 

 
A little bit more refinement around failed transactions needs to be done so the messages are more informative but it does work.
 
Download: pin_payments V1.0.0.zip
 
 

I haven't used the Paypal gateway and I've only just had a quick glance at the code.
 
But you could with the help of the developer documentation do this quite easily, if you open up:
 
/components/gateways/nonmerchant/paypal_payments_standard/paypal_payments_standard.php
if ($amount < 1 && $this->currency = "AUD") || ($amount < 1.20 && $this->currency = "USD") { $this->Input->setErrors($this->getCommonError("unsupported")); } You could probably use a more descriptive error though in this case, but again it's all in the docs.
 
Completely untested, but the above should bring an abrupt end to payments for amounts under $1.00 AUD or $1.20 USD
 
References: 
http://docs.blesta.com/display/dev/Non-merchant+Gateways

I haven't used the Paypal gateway and I've only just had a quick glance at the code.
 
But you could with the help of the developer documentation do this quite easily, if you open up:
 
/components/gateways/nonmerchant/paypal_payments_standard/paypal_payments_standard.php
if ($amount < 1 && $this->currency = "AUD") || ($amount < 1.20 && $this->currency = "USD") { $this->Input->setErrors($this->getCommonError("unsupported")); } You could probably use a more descriptive error though in this case, but again it's all in the docs.
 
Completely untested, but the above should bring an abrupt end to payments for amounts under $1.00 AUD or $1.20 USD
 
References: 
http://docs.blesta.com/display/dev/Non-merchant+Gateways